What You Should Know about Astronomy for the ASVAB

By Rod Powers

You should be equipped with some basic astronomy knowledge for the ASVAB. Earth’s solar system consists of the sun and a number of smaller bodies (such as planets, the planets’ moons, and asteroids) that the sun’s mass holds in orbit. The sun’s mass creates gravity, and this gravity controls the movements of the smaller bodies.

Taking a quick glimpse at the sun

The sun is the largest and most important object in the solar system. It contains 99.8 percent of the solar system’s mass (quantity of matter). The sun provides most of the heat, light, and other energy that makes life possible.

The sun’s outer layers are hot and stormy. The hot gases and electrically charged particles in those layers continually stream into space and often burst out in solar eruptions. This flow of gases and particles forms the solar wind, which bathes everything in the solar system.

The sun is much larger than Earth. The distance from the sun’s center to its surface (the sun’s radius) is about 109 times the radius of Earth. Some of the streams of gas rising from the solar surface are even larger than the Earth’s diameter.

Knowing the planets

A planet is a nonluminous celestial body larger than an asteroid or comet, illuminated by light from a star that the planet revolves around. The solar system consists of eight known planets. In order from closest to the sun to farthest from the sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is no longer classified as a planet by most scientists.

The Earth revolves around the sun in an oval-shaped pattern called an ellipse. Every days, the Earth completes its orbit around the sun and starts again. The Earth rotates (spins) on its axis, completing a rotation every 24 hours, but because of the tilt of the Earth, hours of daylight and darkness aren’t equal, except for on two days a year.

The inner four planets consist chiefly of iron and rock. They’re known as the terrestrial (earthlike) planets because they’re somewhat similar in size and composition. The outer planets are giant worlds with thick, gaseous outer layers. Almost all of their mass consists of hydrogen and helium, giving them compositions more like that of the sun than of Earth. Beneath their outer layers, the giant planets have no known solid surfaces. The pressure of their thick atmospheres turns their insides liquid, though they may have rocky cores.

Rings of dust, rock, and ice chunks encircle all the giant planets. Saturn’s rings are the most familiar, but thin rings also surround Jupiter, Uranus, and Neptune.

Shooting for the moons

Moons (sometimes called satellites) orbit all the planets except Mercury and Venus. The moon you refer to as the moon revolves around the Earth. It makes a complete revolution every days. When the moon moves into the Earth’s shadow, a lunar eclipse results — the Earth is positioned between the sun and the moon. When the Earth moves into the moon’s shadow, a solar eclipse results — the moon is positioned between the Earth and the sun.

The inner planets have few moons. The giant planets probably have more small moons not yet discovered. See the list below for a lineup of the planets and their moons. Although Pluto is no longer officially considered a planet, you never know what those rascally ASVAB test-writers will ask, so Pluto is included in the table.

Planet Number of Moons
Mercury 0
Venus 0
Earth 1
Mars 2 tiny satellites
Jupiter 63
Saturn 61
Uranus 27
Neptune 13
Pluto (dwarf planet) 3

Jupiter’s four largest moons are known as the Galilean satellites because the Italian astronomer Galileo Galilei discovered them in 1610 with one of the first telescopes. The largest Galilean satellite — and the largest satellite in the solar system — is Ganymede, which is even bigger than Mercury and Pluto. The largest of Saturn’s moons, Titan, has an atmosphere thicker than Earth’s and a diameter larger than that of Mercury or Pluto. Pluto’s largest moon, Charon, is more than half the size of Pluto.

Watching for meteors, comets, and asteroids

A meteor is a rock from space that hits Earth’s atmosphere and glows as it heats up, resulting in a brief streak of light. It’s often called a shooting star. When a meteor enters the Earth’s atmosphere, it usually burns up (and that’s a good thing). If a meteor actually strikes the Earth, it’s called a meteorite.

Comets are snowballs composed mainly of ice and rock. When a comet approaches the sun, some of the ice in its nucleus (center) turns into gas. The gas shoots out of the sunlit side of the comet. The solar wind then carries the gas outward, forming it into a long tail. Astronomers divide comets into two main types:

  • Long-period comets, which take 200 years or more to orbit the sun.

  • Short-period comets, which complete their orbits in fewer than 200 years.

The most famous of all comets, Halley’s Comet — also referred to as Comet Halley after Edmond Halley — is a comet that can be seen every 75 to 76 years, making it a short-period comet. Halley is the only short-period comet that is visible to the naked eye and will return within a human lifetime. Its many appearances over the centuries have had a notable effect on human history. Halley’s Comet last appeared in the inner solar system in 1986 and will next appear in mid-2061.

Asteroids are sometimes called minor planets because they’re small bodies that orbit the sun. Some have elliptical orbits that pass inside the orbit of Earth or even that of Mercury. Others travel on a circular path among the outer planets.

Most asteroids circle the sun in a region called the asteroid belt, between the orbits of Mars and Jupiter. The belt contains more than 200 asteroids larger than 60 miles (100 kilometers) in diameter. Scientists estimate that more than 750,000 asteroids with diameters larger than mile (1 kilometer) exist in the belt. There are millions of smaller asteroids, and astronomers have even found several large asteroids with smaller asteroids orbiting them.