Astronomy Study Guide for McGraw-Hill's ASVAB

The Solar System

Earth is the third planet from the sun and, so far as we know, the only planet that currently supports life as we know it. Earth is part of the solar system, a group of planets that revolve around the sun. The planets in order from the nearest to the sun to the furthest, are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. For the ASVAB, it is good to memorize this order. Pluto was recently determined not to be a planet but rather a member of a category of dwarf planets called "plutoids." So now there are only eight planets recognized by scientists.

There is a gap between the orbits of Mars and Jupiter that is filled by tens of thousands of minor planets, known as asteroids. A few asteroids are several hundred kilometers in diameter, but most are just a few meters across. Scientists speculate that these asteroids are part of a planet that failed to form because of Jupiter's strong gravitational pull. Some asteroids have a very elliptical orbit, taking them outside the solar system for part of their orbit. Some asteroids have entered Earth's atmosphere. Generally they burn up before hitting the planet's surface, but a few have collided with the Earth. These are called meteorites. You may have seen some fly through the atmosphere as bright meteors.

Some planets have satellites. Earth has one satellite, called the moon. Jupiter has the most satellites or moons; 39 are currently known. Saturn also has several moons and rings made up of millions of tiny particles. Mars has conditions that are most similar to Earth's, but Mars is not habitable because of its cold temperature and lack of an atmosphere that would support humans. Periodic visitors to the inner solar system are comets, which are believed to be made up of ice and dust particles. Comets have their own orbits. Comets can be recognized by their bright core and tail and can usually be observed for several days.

Motion of the Earth

Seasons   The combination of Earth's revolution around the sun plus the tilt of Earth on its axis causes us to have seasons. Earth revolves around the sun in an elliptical fashion, as shown above. In January, Earth is closest to the sun, at a point called the perihelion. In July, it is farthest from the sun, at a point called the aphelion. On average, Earth is 93 million miles from the sun. The distance from sun to Earth is called an astronomical unit. The distance light travels in one year is called a light-year.

When it is summer in the Northern Hemisphere, Earth's axis points toward the sun at an angle of 23.5°, allowing the sun's rays to hit the northern part of the planet in a more direct fashion and thus raise temperatures in that part of the globe. When it is winter in the Northern Hemisphere, the axis points away from the sun, causing the rays to come in at more of an angle and thus lower temperatures in that part of the globe. The opposite happens in the Southern Hemisphere.

Day and Night   Day and night are caused by Earth's rotation on its axis. One rotation occurs every 24 hours. Days are also longer in the summer and shorter in the winter because of the tilt of Earth's axis. At the vernal and autumnal equinoxes, days and nights are of equal length.

Eclipses   The diagram opposite shows the lineup of the sun, Earth, and moon that results in a lunar eclipse. A solar eclipse occurs when the moon is in between the sun and Earth.