PlanetQuest Glossary

Comets and asteroids
Asteroids are small chunks of mostly rock and metal left over from the formation of the rocky/metallic inner planets—Mercury, Venus, Earth, and Mars. They are found primarily between the orbits of Mars and Jupiter in a region that is known as the asteroid belt. Although there are an estimated 100,000 of them, they don’t really add up to very much. The largest asteroid is only 300 km (200 miles) across, and all of the asteroids together would only make one moderately small moon.

   Comets are small chunks of mostly ice left over from the formation of the gaseous outer planets—Jupiter, Saturn, Uranus, and Neptune. They are found in two regions today. The chunks of ice that originally formed out beyond the orbit of Neptune are still there, in a region known as the Kuiper belt. Meanwhile, those that had originally formed among the outer planets were long ago flung out of the plane of the solar system by gravitational encounters with the planets—primarily Jupiter—and can now be found very far from the Sun in a huge spherical region known as the Oort cloud. Astronomers estimate that as many as a billion comets exist in the Kuiper belt, while the Oort cloud may contain as many as a trillion (1,000 billion). There are a lot of comets out there!
    If there are so many comets, why don't we see them more often? Well, comets spend most of their time either in the Kuiper belt or the Oort cloud. Since they are small and very far away, you would need a very powerful telescope to see them, and if you could see one it would look like nothing more than a chunk of dirty ice a few meters to a few kilometers in size. However, occasionally a comet "falls" toward the Sun and takes a several-months-long trip through the planets. As it nears the Sun, somewhere around the orbit of Mars, it begins to sublimate (sublimation is the process of a solid turning directly into a gas). When this happens, it grows a long "tail" of gas that can be seen streaking outward from the comet. This tail gives the comet its familiar, dramatic appearance and makes it exciting to look at—but you should keep in mind that it is a rare moment of grandeur for the comet.

Earth’s orbit
Once each year—or more precisely, once every 365.25 days—Earth takes one complete trip around the Sun. This motion is known as Earth's orbital motion, or simply its orbit. The shape of Earth's orbital path is an ellipse rather than a perfect circle, which means that its distance from the Sun varies slightly as it goes. However, it is so nearly circular that for most purposes we can assume that Earth's distance from the Sun does not change. This average distance is about 150 million kilometers (93 million miles).
    The distance Earth travels in one orbit is approximately the circumference of a circle with a radius of 150 million kilometers, and so we can figure it out easily enough:

distance = 2p x (150,000,000 km) = 940 million kilometers (580 million miles)

To travel this distance in one year, Earth has to keep moving at a speed of roughly 107,000 kilometers per hour (66,000 miles per hour). Think of that next time you feel like you're not going anywhere!
    Also, since this distance is almost a billion kilometers, or half a billion miles, the next time someone asks you how old you are, you can tell them in kilometers or miles! Instead of saying, "I'm 10 years old," say "I'm 10 billion kilometers old" or "I'm 5 billion miles old." At least you'll give them something to think about!

Earth’s rotation and precession
As Earth travels around the Sun (see Earth's orbit, above) it spins like a top. It spins once every 24 hours, and as it does so it brings you into the side that is lit up by the Sun, and then into the side that is dark (facing away from the Sun), then back into the daylight, then back into darkness again. This is the cause of our 24-hour day-night cycle.
    Because Earth is so large, we don't really feel this spinning motion. Instead, it looks to us as if the entire sky is spinning around us! We see the Sun appear to rise in the east each morning, travel across the sky, and set in the west, only to return and do the same thing the next day, and the next, and the next…. If you are a careful observer, you will also see that the stars rise in the east, travel across the sky, and set in the west each night. All of this apparent motion is due to the real motion of the spinning Earth.
    Just like a top, Earth spins about an axis. This axis runs through the north and south poles of the Earth. This means that if you stood at either the north or south pole at night and looked straight up, it would look to you as if all of the stars in the sky were moving in circles around a point that was straight over your head. It would look as if the stars were all painted onto a huge, domed roof, and that this roof was slowly turning. This point straight over your head is the center point of the apparent rotation of the sky. The point above the North Pole is called the North Celestial Pole, and there happens to be a moderately bright star very close to it. This star is called Polaris, although it is more commonly called the North Star, or the Pole Star. Even if you don't live at the North Pole, if you watch the sky from anywhere in the northern hemisphere it will look as if all of the stars at night are moving in circles around the North Star. The North Star itself will be the one star that will not appear to move—at least, it won't move by very much.
    The point above the South Pole is called the South Celestial Pole, but since there are no bright stars near this point, there is no South Star. This makes simple navigation in the southern hemisphere somewhat more difficult since there are no stars in the sky that do not move during the night.
    If you've ever watched a top spinning on a table top, you have seen that it wobbles. Spinning tops always wobble. This wobbling motion is called precession. Just as tops "precess," so does Earth. Earth's rotation axis "wobbles" slowly as it orbits the Sun. This wobbling is so slow that it takes centuries to have any noticeable effect on what we see in the sky, but what it means is that, even though Polaris is very close to the North Celestial Pole now, it won't always be—and it wasn't always in the past. Astronomers have to take this precession into account when they consider the observations that were made by ancient people, since at the time the observations were made, a different star may have been the Pole Star, and the motion of the sky during the night may have been noticeably different than it is today. See the upcoming article on Namoratunga.

As with every object in the solar system, Earth is always lit up on one side and dark on the other. Since Earth takes 24 hours to turn once on its axis, you might think that we would all have 12 hours of daylight and 12 hours of darkness every day. But this is not the case—at least, not for most people.
    Because Earth’s rotation axis is tipped, people living anywhere other than on the equator will experience more than 12 hours of daylight during the six months that their hemisphere is tipped toward the Sun, and less than 12 hours of daylight during the six months that their hemisphere is tipped away from the Sun (see Earth’s orbit). However, there are two days during the year when every location on Earth experiences exactly 12 hours of daylight and 12 hours of darkness. These days are known as the equinoxes (from the Latin for "equal night").
   The equinoxes occur halfway between the solstices. Since the solstices occur on or near June 21 and December 21, the equinoxes are on or near March 21 and September 21. For folks living in the northern hemisphere, March 21 is known as the spring or vernal equinox, and September 21 is the fall or autumnal equinox. In the southern hemisphere, the designations are reversed. (At the equator, every day has 12 hours of daylight and 12 hours of darkness so the equinoxes make no difference.)
   The equinoxes are also the only two days of the year when the Sun rises directly to the east and sets directly to the west—and this is true everywhere, even at the equator!

The Phases of the Moon
The Sun is the source of essentially all of the visible light in the solar system. Like a giant bonfire surrounded by people, the Sun illumines everything in the solar system that surrounds it. What this means is that every object—every planet, moon, asteroid, comet, etc.—is always light on one side and dark on the other. If from our vantage point here on Earth we sometimes see the side of an object that is lit up, and at other times see the side that is dark, we say that the object goes through phases.
    The Moon is not the only object that goes through phases, but it is the most prominent. Its phases come about because, as it orbits Earth, it is sometimes farther from and sometimes closer to the Sun than we are. When it is farther from the Sun, we are able to see all or part of the side that is lit up. When it is closer to the Sun, some or all of the light side is hidden from our view, and it is the dark side that at least partially faces us.
When the Moon is almost directly between us and the Sun it presents only its dark side to us, and we cannot see it at all. This phase is known as a new Moon. (If the Moon is exactly between us and the Sun there will be a solar eclipse somewhere on the Earth; see Transits, occultations, and eclipses below.) As it continues in its orbit and moves farther from the Sun, we gradually see more and more of its illuminated side. When we can see only a little of it, it looks like a thin crescent in the sky and is known as a crescent Moon, or more specifically as a waxing crescent. (Waxing means it is "getting bigger.") About one week after the new Moon, the Moon will have moved to a place in its orbit where we can see half of the light side and half of the dark side. It then looks like a half circle in the sky and is said to be a first quarter Moon.
    As it continues in its orbit, we see more and more of the light side as it passes through the waxing gibbous phase. Roughly two weeks after the new Moon, the Moon will have moved to a place where it is almost directly opposite the Sun (we are between the Moon and the Sun). We can then see only the side that is lit up, and we call this a full Moon. (If the Moon is exactly opposite the Sun, we see a lunar eclipse; see Transits, occultations, and eclipses below.)
    As it continues in its orbit, we see less and less of the Moon's light side. It passes through the waning gibbous phase until it is once again a half-circle in the sky. This is known as a third quarter Moon. It then shrinks back down, becoming a waning crescent before disappearing completely from our view once again. The entire lunar cycle takes roughly 29.5 days.

The Pleiades
To the naked eye the Pleiades appears as a small, faint but distinct group of stars near the shoulder of the constellation Taurus. It is not itself a constellation to modern day astronomers, but it is one of the most widely known asterisms in the entire night sky. Although on a clear night a person with good vision can generally make out seven stars in the group (which is why it is also known as the "Seven Sisters"), in reality the Pleiades is an open cluster containing several thousand stars, most of which are too faint to be seen without at least a small telescope.

What causes the seasons? That is, why is it hot in the summer and cold in the winter? Many people believe that Earth is hotter in the summertime because it is closer to the Sun. However, this is not so. For one thing, "Earth" is not hotter in the summertime, for whenever the northern hemisphere is experiencing summer, the southern hemisphere is having winter, and vice versa. Furthermore, the variation in Earth's distance from the Sun as it moves in its orbit is too small to have any significant effect on Earth's climate (see Earth's orbit, above).
    The real reason for the seasons is that Earth's rotation axis is tipped (see Earth's rotation, above). This means that sometimes the northern hemisphere is tipped more directly toward the Sun, and sometimes the southern hemisphere is. When your hemisphere is tipped toward the Sun, it's hotter and you call it summer; when your hemisphere is tipped away from the Sun, it's colder and you call it winter.
    By the way, if you were one of those people who thought that the seasons were due to Earth's distance from the Sun, don't feel too bad. A survey of students who were about to graduate from one of the top universities in the world showed that most of them thought so too—as did many of their professors!

The Sidereal and synodic periods of the Moon
The Moon takes approximately 27.3 days to make one trip around the Earth. This is known as the Moon's sidereal period. If the Earth were not moving in its orbit, this would also be the length of the cycle of lunar phases (see above). That is, there would be 27.3 days between one full Moon and the next, or one new Moon and the next. However, because Earth is in orbit around the Sun, the cycle of lunar phases takes 29.5 days. This is known as the Moon's synodic period.
    Why are they different? Well, imagine that the Moon is in its new Moon phase tonight. That means it is between the Earth and the Sun. Now let the Moon continue in its orbit. Obviously, if the Earth didn't go anywhere, the Moon would again be between the Earth and the Sun 27.3 days later. But what happens during those 27.3 days is that the Earth also moves. So when the Moon completes one orbit around the Earth, it is no longer between the Earth and the Sun. It has to continue in its orbit another 2.2 days before it is again in the new Moon phase.
    The synodic period is the time it takes to see the Moon go through one complete cycle of phases. The sidereal period can be seen by watching the Moon against the background stars. If the Moon is in front of one group of stars tonight, then keep an eye on it and you will find that 27.3 days from now it will be in front of the same stars again.

Summer and winter solstices
Because Earth's rotation axis is tilted as it orbits the Sun, there is one day of the year when the North Pole is tipped most directly toward the Sun and the South Pole is tipped most directly away from the Sun. For folks living north of the equator, this is the "longest" day of the year—the day that has the most hours of light. For those living north of the tropics—that is, north of the latitude line known as the Tropic of Cancer—it is also the day during which, at noon, the Sun reaches its highest point above the horizon, and for this reason it is designated as the first day of summer in the northern hemisphere.
    For folks living south of the equator, things are reversed. This is the "shortest" day of the year, and is designated as the beginning of winter. For those living south of the latitude line known as the Tropic of Capricorn, it is also the day during which the Sun reaches its lowest noontime point above the horizon.
    This day—which always occurs on or near June 21—is referred to as the summer solstice by northern hemisphere astronomers, and as the winter solstice by southern hemisphere astronomers. (Since the designation was made before modern astronomy had developed in the southern hemisphere, some southern astronomers still refer to it as the summer solstice, despite the fact that it is the beginning of their winter.)
    Six months later the opposite happens. The Earth passes a point in its orbit where the South Pole is tipped most directly toward the Sun, and the North Pole is tipped most directly away. This day—which always falls on or near December 21—is known as the summer solstice by southern astronomers, and as the winter solstice by northern astronomers.
    By the way, you may be wondering about how the solstices affect people living directly on the equator. Is this the longest, or the shortest day of the year for them? The answer is, neither—and both! The fact is, at the equator every day is the same "length"; it has the same number of hours of light throughout the year, no matter where Earth is in its orbit.

A supernova is an exploding star, and it comes about in one of two ways. When high-mass stars end their "lives"—that is, when they can no longer generate enough energy (through fusion) to balance the immense crushing force of their own gravity—they become unstable and blow up. This is known to astronomers as a "type II supernova," or sometimes as a "massive star supernova." When low-mass stars end their "lives," they do not blow up—at least, not normally. Instead, they leave behind a hot, compact ball of mostly carbon known as a "white dwarf." However, if the white dwarf that's left has a nearby companion (which is actually fairly common), it can draw gas from its companion onto its own surface. If enough of this gas builds up, the white dwarf can become unstable and blow up. This is known as a "type I supernova," or sometimes as a "white dwarf supernova."
    Whichever process is the cause, when a star blows up it is one of the most amazing shows in the Universe. For a few days the explosion of that one star will outshine an entire galaxy of billions of stars. Although astronomers observe supernovae in other galaxies fairly regularly, there has not been one in our own galaxy for several hundred years. A good candidate to keep your eye on, though, is Betelgeuse, the bright reddish star in Orion. Most people refer to Betelgeuse as Orion's left shoulder, although the name actually means "armpit of the Great One." Betelgeuse is known to be nearing the end of its life, so it could go anytime. Maybe tomorrow, maybe next year, but definitely within the next 100,000 years or so.

Transits, occultations, and eclipses
Occasionally when we look at the sky we see one object pass in front of another. If the object that is in front appears small compared with the other, we call this a transit. Mercury passing in front of the Sun is a transit. If the object in front appears very large compared with the other, we call this an occultation. A planet such as Jupiter passing in front of a distant star is an occultation. (In terms of actual size, of course, Jupiter is much smaller than any star, but because it is so much closer to us, it looks bigger.)
    If the two objects—the one in front and the one behind—appear roughly the same size, we call this an eclipse. By this definition the only "proper" eclipse that can be seen from here on Earth occurs when the Moon passes in front of the Sun. This is known as a solar eclipse. However, there is another event that occurs occasionally that is also called an eclipse. When the Moon passes through the Earth's shadow we call this a lunar eclipse. This isn’t a "proper" eclipse, since nothing is blocking our view of the Moon; we are blocking the Moon's view of the Sun. (Notice that every "lunar eclipse" as seen from Earth is a "solar eclipse" as seen from the Moon.)

To astronomers, zenith means directly overhead, or "straight up." When a star passes directly overhead it is said to be at "zenith." Any star that will pass directly overhead at some time during the night is a "zenith star." Which stars are zenith stars depends on where you are on the Earth. For most locations there are many zenith stars. However, at the North Pole there is only one—the North Star (Polaris) is the zenith star 24 hours a day, 365 days a year.
    For those who live in the tropics—that is, in latitudes between the Tropic of Cancer (23.5 degrees north of the equator) and the Tropic of Capricorn (23.5 degrees south of the equator)—the Sun will pass directly overhead on two days during the year. (If you live exactly on the Tropic of Cancer this will happen only once—on the summer solstice. Likewise, if you live exactly on the Tropic of Capricorn it will happen only on the winter solstice.) This event is sometimes referred to as a "zenith Sun."