Astronomy Articles

Article / Updated 02-28-2025

Just as you can mark the Moon’s position each night against the fixed stars and constellation patterns, you can mark the planets, too. The planets also make paths against the fixed stars. Over the course of a year, these paths join into one line that circles the entire sky. This line is known as the zodiac, and its more scientific name is the ecliptic. This ecliptic line passes through the signs of the zodiac, which comprises 12 familiar constellations as shown in the figure: Aries Taurus Gemini Cancer Leo Virgo Libra Scorpius Sagittarius Capricornus Aquarius Pisces Contrary to popular belief, however, the zodiac actually has 13 signs, not 12. The thirteenth sign is called Ophiuchus the Serpent-bearer. Don’t worry if you’ve never heard of him – most people haven’t! He sits between Scorpius and Sagittarius, and so the Sun, Moon and planets all spend a bit of time in Ophiuchus each time they move around the sky. Whenever you see a planet, the Sun or the Moon in the sky, it always lies somewhere in one of the signs of the zodiac. You can find out where a planet is on the ecliptic by looking it up online – a search for ‘Jupiter position April’ should give you lots of websites that will tell you where to look for Jupiter in April. Alternatively, you may want to subscribe to an astronomy magazine. These monthly publications list all the planets that are up in the sky for the particular month, and feature maps that show you where to find them.

Cheat Sheet / Updated 07-03-2024

Stargazing is a fascinating hobby, but there’s an awful lot to gaze at up there. Eighty-eight constellations and hundreds of other objects both bright and faint mean that wherever you look when you’re stargazing, there’s something to see. If you want to make sense of it all, and make sure that your kit is properly set up, this Cheat Sheet is here to help.

Article / Updated 10-19-2023

The Space Age, generally considered started by the launch of the Russian satellite Sputnik, is defined by the events surrounding space exploration and development of space technology. This list maps out major events of the Space Age: 1957 The Soviet Union launches Sputnik 1, the first artificial satellite to orbit Earth. 1958 Using the satellite Explorer 1, James Van Allen discovers Earth’s radiation belts (magnetosphere). 1960 Frank Drake begins the Search for Extraterrestrial Intelligence (SETI) at the National Radio Astronomy Observatory in Green Bank, West Virginia. 1961 Yuri Gagarin makes the first manned space flight. 1963 Valentina Tereshkova is the first woman in space. 1967 Jocelyn Bell Burnell and Anthony Hewish discover pulsars. 1969 Neil Armstrong and Buzz Aldrin walk on the moon. 1971: The Soviet Union launches the first space station, Salyut 1. 1972: The Soviet Union's Mars 3 spacecraft makes first soft landing on another planet, Mars. 1979 Using pictures from Voyager 1, Linda Morabito discovers erupting volcanoes on Jupiter’s moon, Io. 1981: NASA's Columbia shuttle becomes first winged spaceship to orbit Earth and return to airport landing. 1987 Ian Shelton discovers the first supernova since 1604 plainly visible to the naked eye. 1990 The Hubble Space Telescope launches. 1991 Alexander Wolszczan discovers planets orbiting a pulsar — the first known planets outside the solar system. 1995 Michel Mayor and Didier Queloz discover 51 Pegasi B, the first planet of a normal star beyond the Sun. 1998 Two astronomer teams discover that the expansion of the universe is getting faster, perhaps due to a mysterious “dark energy” associated with the vacuum of space. 1999 Mars Global Surveyor finds that Mars may have had an ocean at one time. 2003 The Wilkinson Microwave Anisotropy Probe satellite finds that the universe is 13.7 billion years old. 2012 The Kepler spacecraft finds that there probably are billions of planets in orbit around stars in our galaxy, and the rover Curiosity lands on Mars. 2015 The New Horizons probe explores Pluto and its moons and then heads outward in the Kuiper Belt.

Article / Updated 10-19-2023

A comet is a stuck-together mixture of ice, frozen gases (such as the ices of carbon monoxide and carbon dioxide), and solid particles — the dust or "dirt" shown here. Historically, astronomers described comets as having a head and tail or tails, but with additional research, they've been able to clarify the nature of a comet's structure. The nucleus Astronomers initially named a bright point of light in the head of a comet the nucleus. Today we know that the nucleus is the true comet — the so-called dirty ice ball. The other features of a comet are just emanations that stem from the nucleus. A comet far from the Sun is only the nucleus; it has no head or tail. The ice ball may be dozens of miles in diameter or just a mile or two. That size is pretty small by astronomical standards, and because the nucleus shines only by the reflected light of the Sun, a distant comet is faint and hard to find. Images of Halley's nucleus from a European Space Agency probe that passed very close to it in 1986 show that the lumpy, spinning ice ball has a dark crust, like the tartufo dessert (balls of vanilla ice cream coated with chocolate) served in fancy restaurants. Comets aren't so tasty, but they are real treats to the eye. Here and there on Halley's nucleus, the probe photographed plumes of gas and dust from geyserlike vents or holes, spraying into space from areas where the Sun was warming the surface. Some crust! And in 2004, NASA's Stardust probe got close-up images of the nucleus of Comet Wild-2. This nucleus seems to bear impact craters and is marked with what may be pinnacles made of ice. Those are the cold facts. Not all comet nuclei are shaped like Halley's, though. In August 2014, the Rosetta spacecraft reached Comet 67P/Churyumov–Gerasimenko, known as 67P to its friends (like me). Rosetta orbited the comet nucleus while the comet orbited the Sun until the end of the European Space Agency mission in September 2016. Its photographs revealed a nucleus shaped roughly like a dumbbell with two unequal weights. Astronomers referred to the "weights" as two lobes of the comet connected by a thinner structure they named the neck. Some astronomers stuck their necks out by theorizing that the odd-shaped nucleus was formed by the low-speed collision of two earlier objects. The coma As a comet gets closer to the Sun, solar heat vaporizes more of the frozen gas, and it spews out into space, blowing some dust out, too. The gas and dust form a hazy, shining cloud around the nucleus called the coma (a term derived from the Latin for "hair," not the common word for an unconscious state). Almost everyone confuses the coma with the head of the comet, but the head, properly speaking, consists of both the coma and the nucleus. The glow from a comet's coma is partly the light of the Sun, reflected from millions of tiny dust particles, and partly emissions of faint light from atoms and molecules in the coma. A tale of two tails The dust and gas in a comet's coma are subject to disturbing forces that can give rise to a comet's tail(s): the dust tail and the plasma tail. (Sometimes when you view a comet, you see just one kind of tail, but when you're lucky, you see both.) The pressure of sunlight pushes the dust particles in a direction opposite the Sun, producing the comet's dust tail. The dust tail shines by the reflected light of the Sun and has these characteristics: A smooth, sometimes gently curved appearance A pale yellow color The other type of comet tail is a plasma tail (also called an ion tail or a gas tail). Some of the gas in the coma becomes ionized, or electrically charged, when struck by ultraviolet light from the Sun. In that state, the gases are subject to the pressure of the solar wind, an invisible stream of electrons and protons that pours outward into space from the Sun. The solar wind pushes the electrified cometary gas out in a direction roughly opposite of the Sun, forming the comet's plasma tail. The plasma tail is like a wind sock at an airport: It shows astronomers who view the comet from a distance which way the solar wind is blowing at the comet's point in space. In contrast to the dust tail, a comet's plasma tail has the following: A stringy, sometimes twisted, or even broken appearance A blue color Now and then, a length of plasma tail breaks from the comet and flies off into space. The comet then forms a new plasma tail, much like a lizard that grows a new tail when it loses its first one. The tails of a comet can be millions to hundreds of millions of miles long. When a comet heads inward toward the Sun, its tail or tails stream behind it. When the comet rounds the Sun and heads back toward the outer solar system, the tail still points away from the Sun, so the comet now follows its tail. The comet behaves to the Sun as an old-time courtier did to his emperor: never turning his back on his master. The comet shown could be going clockwise or counterclockwise, but either way, the tail always points away from the Sun. The coma and tails of a comet are just a vanishing act. The gas and dust shed by the nucleus to form the coma and tails are lost to the comet forever — they just blow away. By the time the comet travels far beyond the orbit of Jupiter, where most comets come from, it consists of only a bare nucleus again. And the nucleus is a little smaller, due to the gas and dust that it sheds. The dust the comet loses may someday produce a meteor shower, if it crosses Earth's orbit. Halley's comet is a good example of the wasting-away process. Halley's nucleus decreases by at least a meter (39.37 inches, or slightly more than a yard) every 75 to 77 years when it passes near the Sun. The nucleus is only about 10 kilometers (10,000 meters or 6.2 miles) in diameter right now, so Halley's comet will survive only about 1,000 more orbits, or about 75,000 years. Dust shed by the famous comet causes two of the top annual meteor showers, the Eta Aquarids and the Orionids.

Article / Updated 10-19-2023

If you're just starting to become interested in astronomy, get into the astronomy hobby gradually, investing as little money as possible until you're sure about what you want to do. Here's a plan for acquiring both basic skills and the needed equipment: If you have a late-model computer, invest in a free or inexpensive planetarium program. Better yet, if you have a smartphone, download and use a free or cheap planetarium app. Start making naked-eye observations at dusk on clear nights and before dawn, if you're an early riser. To plan your observations of planets and constellations, you can also rely on the weekly sky scenes at the Sky & Telescope website. If you don't have a suitable computer, plan your observations based on the monthly sky highlights in Astronomy or Sky & Telescope magazine. After a month or two of familiarizing yourself with the sky and discovering how much you enjoy it, invest in a serviceable pair of 7x50 binoculars. As you continue to observe the bright stars and constellations, invest in a star atlas that shows many of the dimmer stars, as well as star clusters and nebulae. Sky & Telescope's Pocket Sky Atlas by Roger W. Sinnott (Sky Publishing, 2007) is a good choice. For maps that are equally good but larger, consult the Jumbo Pocket Sky Atlas by the same author and publisher (2016); you'll just need a bigger pocket. Compare scenes in your star atlas with the constellations that you're observing; the atlas shows their RAs and Decs. Eventually, you'll start to develop a good feel for the coordinate system. Join an astronomy club in your area, if at all possible, and get to know the folks who have experience with telescopes. If all goes well and you want to continue in astronomy invest in a well-made, high-quality telescope in the 2.5-to-4-inch size range. Study the telescope manufacturer websites earlier in this chapter or send for catalogs advertised in astronomy magazines. Better yet, talk to experienced astronomy club members if you can. They can advise you on buying a new telescope, and they may know someone who wants to sell a used telescope. You may be able to borrow a starter telescope and try it out at home. Thanks to the New Hampshire Astronomical Society (NHAS), a movement to place such telescopes in public libraries has begun. Astronomy clubs purchase the telescopes; club members modify them for use by inexperienced borrowers and then donate them to the libraries. The telescope model adopted for this project is the Orion StarBlast 4.5, which retails for about $210. It's meant for use on a tabletop but may work for you when just placed on the ground. According to Sky & Telescope, by late 2016, NHAS had placed more than 100 of these telescopes in New Hampshire libraries, and the St. Louis Astronomical Society had placed over 130 in Missouri and Illinois libraries. Astronomy clubs in other areas are beginning to sponsor library telescopes; search the web to see whether a library telescope program exists near you. Who knows; you may have a (star) blast! If you find that you enjoy astronomy, after a few years, consider moving up to a 6- or 8-inch telescope. It may be harder to use, but you'll be ready to master it after you have some experience. Equipped with a larger telescope, you can see many more stars and other objects. You can get ideas about what larger telescopes to consider by talking to other amateur astronomers and by attending a star party, where you can see many different telescopes in operation and on display.

Article / Updated 10-19-2023

Digital cameras are now the preferred tools for photographing meteors. But digital meteor photography requires a digital single lens reflex camera (DSLR), which is an expensive camera (point-and-shoot cameras and cellphone cameras don't work very well, except in the rare case when you can catch a brilliant fireball) and a lot of trial-and-error experimenting until you get it right. Further, you need a DSLR that you can set for time exposures and that accepts a cable for an intervalometer or "remote switch with digital timer." You might need to spend more on a suitable camera for meteor photography than on a decent small telescope for other observations, but the camera can be used for other purposes, not just your astronomy hobby. Here are some important guidelines for digital meteor photography: Observe from as dark a location as possible, away from urban lighting. Try meteor photography only when the Moon is below the horizon. Use a sturdy tripod so the camera doesn't shake during a time exposure. Use a wide-angle lens (because you'll catch more meteors in a single shot than with a normal lens) and set it on Infinity. Don't use a telephoto lens. Use an intervalometer or "remote switch with digital timer" to operate the camera shutter without shaking the camera and to take pictures at regular intervals during the night. Point the camera about halfway up the sky from the horizon to the zenith, or a little higher, facing whichever direction has the least interfering sky glow from city or other lights. Spend some time making test exposures to determine what settings to use on that particular night. (The best settings vary depending on how bright the sky is.) Make several 10-second exposures, some 20-second exposures, and some 30-second exposures. You're trying to determine how long you can let an exposure last (the longer the better) without skylight overexposing the picture. You may need to repeat this series of time exposures for each of two or three ISO settings. (With a larger ISO setting, you can record fainter meteors, which means more meteors, but with the larger ISO setting, the sky overexposes sooner, so you can't expose for as long a time.) With experience, you should find the "sweet spot" of exposure time and ISO that works best with your lens at your location. For more info on digital meteor photography read this expert advice. You can photograph sporadic meteors by following the preceding guidelines, but there aren't many sporadic meteors to catch on any given night. A meteor shower offers you the opportunity to snap more meteors, as long as the Moon isn't in the sky. With moonlight, you'll catch far fewer meteors, if any. When photographing a meteor shower, take the photographs when the shower radiant (the constellation from which the meteor shower seems to come) is well above the horizon, preferably 40 degrees or more. The horizon is at 0 degrees altitude, and the zenith (overhead point) is 90 degrees up, so the halfway point between them is at 45 degrees; two-thirds of the way up is 60 degrees, and so on.

Article / Updated 10-19-2023

Hundreds of operating satellites are orbiting Earth, along with thousands of pieces of orbiting space junk — nonfunctional satellites, upper stages from satellite launch rockets, pieces of broken and even exploded satellites, and tiny paint flakes from satellites and rockets. You may be able to glimpse the reflected light from any of the larger satellites and space junk, and powerful defense radar can track even very small pieces. The best way to begin observing artificial satellites is to look for the big ones — such as NASA's International Space Station or the Hubble Space Telescope — and the bright, flashing ones (the dozens of Iridium communication satellites). Looking for a big or bright artificial satellite can be reassuring to the beginning astronomer. Predictions of comets and meteor showers are sometimes mistaken, the comets usually seem fainter than you expect, and usually you see fewer meteors than advertised. But artificial satellite viewing forecasts are usually right on. You can amaze your friends by taking them outside on a clear early evening, glancing at your watch, and saying "Ho hum, the International Space Station should be coming over about there (point in the right direction as you say this) in just a minute or two." And it will! Want to know what to watch for? Here are some characteristics you can pinpoint for both large and bright satellites: A big satellite such as the Hubble Space Telescope or the International Space Station generally appears in the evening as a point of light, moving steadily and noticeably from west to east in the western half of the sky. It moves much too slowly for you to mistake it for a meteor, and it moves much too fast for a comet. You can see it easily with the naked eye, so it can't be an asteroid — and, anyway, it moves much faster than an asteroid. Sometimes you may confuse a high-altitude jet plane with a satellite. But take a look through your binoculars. If the object in view is an airplane, you should be able to distinguish running lights or even the silhouette of the plane against the dim illumination of the night sky. And when your location is quiet, you may be able to hear the plane. You can't hear a satellite. An Iridium satellite is a wholly different viewing situation: It usually appears as a moving streak of light that gets remarkably bright and then fades after several seconds. It moves much more slowly than a meteor. And an Iridium flare or flash is often brighter than Venus, second in brilliance only to the Moon in the night sky. The Sun, located below your horizon, reflects off one of the door-size, flat, aluminum antennas on the satellite to cause the flash of light. At star parties, people cheer when they spot an Iridium flare, just like when folks see a fireball. You can even see some Iridium flares in daylight. And consider this: More than 60 Iridium satellites are in orbit. They interfere with astronomy, and professional astronomers want them to disappear, but until now, at least, the satellites have had a "flare" for entertaining us. A subsequent generation of the satellites, called Iridium NEXT, started being launched into space in January 2017. The NEXT satellites may be next to useless for amateur flare watchers because the design of the antennas has changed so that bright reflections from them are unlikely. The good news is that retiring all the original Iridiums will take a while, so if you start looking soon you may be able to catch some impressive flares before they're just history.

Article / Updated 10-19-2023

When you're outdoors on a dark night and see a "shooting star" (the flash of light from a random, falling meteoroid), what you're probably seeing is a sporadic meteor. But if many meteors appear, all seeming to come from the same place among the stars, you're witnessing a meteor shower. Meteor showers are among the most enjoyable sights in the heavens. A dazzlingly bright meteor is a fireball. Although a fireball has no official definition, many astronomers consider a meteor that looks brighter than Venus to be a fireball. However, Venus may not be visible at the time you see the bright meteor. So how can you decide whether you're seeing a fireball? Here's a rule for identifying fireballs: If people facing the meteor all say "Ooh" and "Ah" (everyone tends to shout when they see a bright meteor), the meteor may be just a bright one. But if people who are facing the wrong way see a momentary bright glow in the sky or on the ground around them, it's the real thing. To paraphrase an old Dean Martin tune, when the meteor hits your eye like a big pizza pie, that's a fireball! Fireballs aren't very rare. If you watch the sky regularly on dark nights for a few hours at a time, you'll probably see a fireball about twice a year. But daylight fireballs are very rare. If the Sun is up and you see a fireball, mark it down as a lucky sighting. You've seen one tremendously bright fireball. When nonscientists see daytime fireballs, they almost always mistake them for an airplane or missile on fire and about to crash. Any very bright fireball (approaching the brightness of the half Moon or brighter) or any daylight fireball represents a possibility that the meteoroid producing the light will make it to the ground. Freshly fallen meteorites are often of considerable scientific value, and they may be worth good money, too. If you see a fireball that fits this description, write down all the following information so your account can help scientists find the meteorite and determine where it came from: Note the time, according to your watch. At the earliest opportunity, check how fast or slow your watch is running against an accurate time source, such as the Official U.S. Time site. If you have a smartphone, it should give you the time accurate at least to the minute. Record exactly where you are. If you have a Global Positioning System receiver handy (or a smartphone with a GPS app, such as Compass on the iPhone), take a reading of your latitude and longitude. Otherwise, make a simple sketch showing where you stood when you saw the fireball — note roads, buildings, big trees, or any other landmarks. Make a sketch of the sky, showing the track of the fireball with respect to the horizon as you saw it. Even if you're not sure whether you faced southeast or north-northwest, a sketch of your location and the fireball track helps scientists determine the trajectory of the fireball and where the meteoroid may have landed. After a daylight fireball or a very bright nighttime fireball, interested scientists advertise for eyewitnesses. They collect the information, and by comparing the accounts of persons who viewed the fireball from different locations, they can close in on the area where it most likely fell to the ground. Even a brilliant fireball may be only the size of a small stone — one that would fit easily in the palm of your hand — so scientists need to narrow the search area to have a reasonable chance of finding it. If you don't see a call for information after your fireball observation, chances are good that the nearest planetarium or natural history museum will accept your report and know where to send it. Or, report your fireball observation to the American Meteor Society — just look for the prominent "Report a Fireball" link on their home page. A bolide is a fireball that explodes or produces a loud noise even if it doesn't break apart. Some people use bolide interchangeably with fireball. (You won't find an official agreement on this term; you can find different definitions in even the most authoritative sources.) The noise you hear is the sonic boom from the meteoroid, which is falling through the air faster than the speed of sound. When a fireball breaks apart, you see two or more bright meteors at once, very close to each other and heading the same way. The meteoroid that produces the fireball has fragmented, probably from aerodynamic forces, just as an airplane falling out of control from high altitude sometimes breaks apart even though it hasn't exploded. Often a bright meteor leaves behind a luminous track. The meteor lasts a few seconds or less, but the shining track — or meteor train — may persist for many seconds or even minutes. If it lasts long enough, it becomes distorted by the high-altitude winds, just as the wind gradually deforms the skywriting from an airplane above a beach or stadium. You see more meteors after midnight local time than before because, from midnight to noon, you're on the forward side of Earth, where our planet's plunge through space sweeps up meteoroids. From noon to midnight, you're on the backside, and meteoroids have to catch up in order to enter the atmosphere and become visible. The meteors are like bugs that splatter on your auto windshield. You get many more on the front windshield as you drive down the highway than on the rear windshield because the front windshield is driving into bugs and the rear windshield is driving away from bugs.

Article / Updated 09-14-2023

Here are some favorite facts about astronomy and, in particular, Earth and its solar system. With the following information under your belt, you may be ready to handle the astronomy questions on television quiz shows and inquiries from friends and family. You have tiny meteorites in your hair Micrometeorites, tiny particles from space visible only through microscopes, are constantly raining down on Earth. Some fall on you whenever you go outdoors. But without the most advanced laboratory equipment and analysis techniques, you can't detect them. They get lost in the great mass of pollen, smog particles, household dust, and dandruff that resides on the top of your head. A comet's tail often leads the way A comet tail isn't like a horse tail, which always trails behind as the horse gallops ahead. A comet tail always points away from the Sun. When a comet approaches the Sun, its tail, or tails, stream behind it; when the comet heads back out into the solar system, the tail leads the way. Earth is made of rare and unusual matter The great majority of all the matter in the universe is so-called dark matter, invisible stuff that astronomers haven't yet identified. And most ordinary or visible matter is in the form of plasma (hot, electrified gas that makes up normal stars such as the Sun) or degenerate matter (in which atoms or even the nuclei within the atoms are crushed together to unimaginable density, as found in white dwarfs and neutron stars). You don't find dark matter, degenerate matter, or much plasma on Earth. Compared to the great bulk of the universe, Earth and earthlings are the aliens. High tide comes on both sides of Earth at the same time Ocean tides on the side of Earth that faces the Moon aren't appreciably higher than tides on the opposite side of Earth at the same time. This may defy common sense, but not physics and mathematical analysis. (The same goes for the smaller ocean tides raised by the Sun.) On Venus, the rain never falls on the plain In fact, the constant rain on Venus never falls on anything. It evaporates before it hits the ground, and the rain is pure acid. (The common name for evaporating rain is virga.) Rocks from Mars dot Earth People have found about 100 meteorites on Earth that come from the crust of Mars, blasted from that planet by the impacts of much larger objects — perhaps from the asteroid belt. Statistically, many more undiscovered Mars rocks must have fallen into the ocean or landed in out-of-the-way places where they haven't been spotted. Pluto was discovered from predictions of a false theory Percival Lowell predicted the existence and approximate location of the object that we now call Pluto. When Clyde Tombaugh surveyed the designated region, he discovered Pluto. But now scientists know that Lowell's theory, which inferred the existence of Pluto from its gravitational effects on the motion of Uranus, was wrong. In fact, Pluto's mass is much too small to produce the "observed" effects. Furthermore, the "gravitational effects" were just errors in measuring the motion of Uranus. (Not enough information was available about Neptune's motion to study it for clues.) The discovery of Pluto took hard work, but as it happened, it was just plain luck. And although Lowell predicted the existence of a planet, as Pluto was first termed, the International Astronomical Union has since downgraded it to dwarf planet. Sunspots aren't dark Almost everyone "knows" that sunspots are "dark" spots on the Sun. But in reality, sunspots are simply places where the hot solar gas is slightly cooler than its surroundings. The spots look dark compared to their hotter surroundings, but if all you can see is the sunspot, it looks bright. A star in plain view may have exploded, but no one knows Eta Carinae is one of the most massive, fiercely shining stars in our galaxy, and astronomers expect it to produce a powerful supernova explosion at any time, if it hasn't already. But because light takes about 8,000 years to travel from Eta Carinae to Earth, an explosion that occurred many years ago isn't visible to us yet. You might have seen the Big Bang on an old TV set The Big Bang Theory premiered in 2007, but the real Big Bang may have made its TV debut even before that. Some of the snow — a pattern of interference that looks like little white spots or streaks on old black-and-white television sets — was actually radio waves the TV antenna received from the cosmic microwave background, a glow from the early universe in the aftermath of the Big Bang. When this radiation was actually discovered at the Bell Telephone Laboratories, scientists studied many possible causes of the unexpected "noise" in the radio receiver. They even investigated pigeon droppings, or "white dielectric material" in science speak, as a possible cause but later dropped that suggestion.

Article / Updated 09-05-2023

You’ve probably noticed that September just “feels” like a time of transition. For those of us in the Northern Hemisphere, it’s the time of the year when temperatures cool off, birds begin gathering for migration, and the trees start showing color. All of this occurs around the autumnal equinox, between September 21 and September 24 (in 2023, the equinox is the 23rd) marking the start of fall for people in the Northern Hemisphere. Earth’s tilt and orbit make it all happen Our planet is tilted about 23.5 degrees on its axis as it travels around the sun, and it stays tilted in the same direction as it orbits. This means the sun’s light hits the Earth at different angles. So, different parts of the Earth receive varying amounts of the sun’s light and warmth throughout the year — in other words, we have seasons. However, twice during Earth’s orbit, on the autumnal (September) equinox and the vernal, or spring (March), equinox, the sun is directly over the planet’s equator, and everybody across the globe experiences a nearly equal amount of daylight and darkness. A few interesting facts about the equinoxes and Earth’s orbit: The word equinox comes from two Latin words: aequus (equal) and nox (night). During the autumnal and vernal equinoxes, people across the globe can see the sun rise above the horizon due east and set due west. So, it’s a good time to find due east and west. Because of the Earth’s tilt on its axis, its northern and southern hemispheres trade places throughout the year receiving the sun’s light and warmth most directly. For people in the Southern Hemisphere, the equinoxes signal the opposite seasonal transitions: September marks the beginning of spring, and the March equinox marks the beginning of autumn. If you live in the Northern Hemisphere, starting on the autumnal equinox, you can observe a slight daily change in the sun’s arc across the sky — it’s shifting to the south. A number of prehistoric sites, like Stonehenge and Newgrange in the United Kingdom, were possibly used by ancient cultures to predict equinoxes and the summer and winter solstices — the longest and shortest days of sunlight during the year). To learn more about the Earth's orbit, our solar system, the stars, and beyond, check out the fifth edition of Astronomy For Dummies. Celebrations around the fall equinox Over the centuries, before humans could scientifically explain the autumnal equinox, they observed the sun’s changing position in the sky and nature's transitions in the fall and spring, and they marked these times with rituals and celebrations. Some of these ancient observations were incorporated into Greek mythology and other cultures' mythology, and into religious practices. Today, there are still many cultural and religious traditions practiced around the equinoxes. Here are summaries of just a few: Mabon — United Kingdom: Mabon is a fall equinox tradition created by the ancient Celtic people and celebrated by pagans today. It is one of the oldest harvest festivals in Europe. Acknowledging the autumnal equinox, the holiday is meant to give thanks for the warm, summer months, the fall harvest, and to get ready for the beginning of winter. The Snake of Sunlight — Mexico: The ancient Mayan and Aztec civilizations in Mexico celebrated the equinoxes at the site of Chichen-Itza, a city that existed about 1,500 years ago in what is now the state of Yucatan. When you visit the Chichen-Itza ruins today, you see a massive pyramid, a monument the Mayans built to honor the god Kulkulcan (Quetzalcoatl to the Aztecs). The deity was a feathered serpent, and the ancient Mayans believed it visited the temple twice a year — on the autumn and spring equinoxes. Chichen-Itza is a popular tourist attraction, and many come to witness a special effect that happens on the equinoxes. The pyramid’s steps are oriented so that in the afternoon of the two equinox days, the shadow on the pyramid looks like a snake slowly slithering down the stairs, with its tail at the top and its head at the bottom. Higan — Japan: Higan is a Buddhist tradition taking place around the equinoxes – three days before the equinox day and three days after. Higan means crossing over to the “other shore,” symbolizing the world of enlightenment, or spiritual awakening. For Buddhists, Higan is a time to reflect on one’s life and renew religious practices. During Higan in Japan, people visit their ancestors’ graves, where they tidy up the gravesites and place flowers and incense.