Watching the Planets
As we learn more and more about the planets, our fascination with them only grows. The planets are one of the most visible elements of the night sky, next to the Moon. Even with the naked eye they make a magnificent sight, and viewing through binoculars or a telescope only increases their magnificence.
Planetary Observation Guide
There are some important terms that are used when discussing the movement of the planets, and when they are observable to we terrestrial viewers.
- Inferior Planets: The planets which orbit closer to the Sun than the Earth – Mercury and Venus
- Superior Planets: The planets whose orbits are further from the Sun that the Earth’s – Mars, Jupiter, Saturn, Uranus, Neptune & Pluto
- Elongation: The arc angle degree that separates a planet from the Sun. Most often used to discuss the inferior planets
- Conjunction: When a planet is completely in line with another planet, moon or the Sun
- Opposition: When a superior planet is on the opposite side of the Earth from the Sun, and is at its closest point to the Earth.
- Retrograde Motion: Normally, the planets appear to move across our sky in a west to east direction. However, sometimes they seem to move in the opposite direction, moving west. This occurs when Earth is passing them in orbit: the Earth’s orbit is faster than the ther planets, so they appear to be receding in the night sky, but we are actually just passing them.
When observing the inferior planets, the time of best viewing is at greatest eastern or western elongation: when they are the furthest away from the Sun in either direction. For the superior planets, the best time for viewing is during opposition: when they are closest to Earth and on the direct opposite side of the Earth from the Sun, so that we might see all of the planet.
Finding Periods of Opposition and Elongation
In order to properly view the planets, you are going to need to know when they are in periods of opposition or greatest elongation. There are many almanacs that give these periods, projecting years or even decades in advance. You can purchase these books, or you can often also find these books at your local library.
Astronomy magazines are another great way to keep up with the movement of the planets, and the rest of the night sky. They can be found in most book stores.
If you aren’t interested in books, there are plenty of places on the internet where you can get up to date viewing on the movements of the planets. Unlike other natural phenomena the movement of the planets are constant and can be predicted perfectly, so you know that your information will never lead you wrong.
You will also find all of these sources invaluable in telling you where exactly in the sky to look for these celestial bodies.
Mercury
Next to Pluto, Mercury is the smallest planet in our solar system at only 3,000 miles diameter. It is also the closest to our Sun. This combination of small size and proximity to the Sun makes it very difficult to see in detail.
This does not mean that we cannot see Mercury, however. Mercury can be seen at any elongation, and it goes through elongation every couple of months. The viewing window lasts from about a week before the date of greatest elongation to a week after.
Because Mercury, as well as Venus, are closer to the Sun than the Earth, we can actually see its phases, like the phases of the moon. When at greatest elongation, both Mercury and Venus appear as half-full.
When Mercury is in greatest eastern elongation, it is best to view it in the evening, about a half hour after sunset.
When Mercury is in greatest western elongation, the time for viewing is in the morning. Start watching about 15 minutes before the sunrise.
During both eastern and western elongations, Mercury will appear as a small, bright star. It is close to the horizon, and will most likely be the only thing that you can see in the sky, so it will not be difficult to recognize it for Mercury.
Venus
Venus is the planet that comes closest to earth, within 26 million miles. It is well over twice the size of Mercury, and is much easier for us to see. Like Mercury it is most visible in the morning or in the evening, and in fact in ancient times it was called the Morning Star or the Evening Star. (Most ancient peoples did not recognize them as the same star, although this was not always true).
It is difficult to see much detail of the surface of Venus, as it also is for Mercury. Because they are so close to the Sun, we normally see both of these planets only as balls of light, although we are able to tell that they are planets.
Venus is the brightest of all the planets, reaching a magnitude of -4.7. Because of its brightness, Venus is very easy to spot in the sky. It is the first star to appear at night when visible, and is visible long after the other stars have faded in the morning.
Mars
Of all the planets of our solar system, it is Mars that most holds our attention. It is the second-closest planet to us after Venus, but it is the planet that is most like our own. It is the only other planet in our solar system that could possibly sustain life, and we are always searching for evidence that life has existed there.
Unlike Mercury and Venus, astronomers are actually able to distinguish many of the features of Mars’s surface, even with an
ordinary telescope. This is another of the reasons why Mars has so fascinated us: because unlike other planets, we are actually able to examine its surface.
In the 19th century, grand stories of Martian canals, built by some sophisticated Martian race, captured the imaginations of people the world over. H.G. Welles’s War of the Worlds featured an invasion of the Earth by Martian creatures with technology far more advanced than our own.
During the 20th century these myths were dispelled, the idea of canals and an advanced Martian race. However, it did not lessen our fascination with the red planet at all. We still look to it as our most neighborly neighbor, the planet besides our own that might hold the wonders of life, and might one day become our home.
Unfortunately for Mars viewers, it is only possible to see Mars every couple of years or so. The cycle of opposition takes about 26 months for Mars to complete. It is also closer to the Earth during some oppositions than others.
In August of 2003, Mars came closer to the Earth than it had for 60,000 years. While it will still come close to the Earth every couple of years, it will be quite some time before it gets that close again.
Viewing of Mars is best starting about one month before the date of opposition to a month after. It is best to view Mars when it is highest in the sky: this comes at about midnight on the day of opposition. It might require staying up a little bit late, but it will produce spectacular views of Mars.
To the naked eye, Mars appears to be a large, bright red star. Using a telescope, it is possible to see the surface of the planet and begin investigating its surface in a way not possible with the inferior planets.
Jupiter
The next in the planetary line is Jupiter. Jupiter is by far the largest planet in our solar system, almost 89,000 miles in diameter (almost 30 times the size of Mercury!). The most famous part of Jupiter is the great red spot, a fierce storm that constantly rages upon the surface of the planet. This is only one of the many features which are visible on Jupiter, making it a fascinating to study through a telescope.
You might notice, though, when viewing Jupiter through a telescope, that the famous great red spot is not in the same place it is usually shown to be in pictures and movies, but will instead be in the northern hemisphere rather than the south. That is because the telescope, unless you have a special eyepiece, will invert the image that you are seeing.
Jupiter is a very different planet from our own. It is one of the gas giants (the others are Saturn, Uranus and Neptune). Although it has a metallic core, the planet consists primarily of great clouds of hydrogen, helium, and trace amounts of other gases.
Not only is Jupiter visible in the night sky, but with even a pair of binoculars some of its moons are visible as well. Although Jupiter has a total of 16 moons, there are 4 of these that are easily visible to us. They are referred to as the Galilean moons because they were first observed by Galileo when he first turned his telescope towards the stars.
The four Galilean moons are:
- Io: Closest of the Galilean moons to Jupiter
- Ganymede: Largest moon in the solar system, second closest of the Galilean moons.
- Europa: The smallest of Jupiter’s moons, third closest.
- Callisto: The most distant of the Galilean moons from Jupiter.
Jupiter, unlike the other planets, does not experience drastic changes in relative size for viewers from Earth. Of course it appears largest and brightest during opposition, but it can easily be found at all times in the night sky.
Saturn
Saturn is most famous for its rings. Uranus and Neptune also have rings, but they are not nearly as large or as visible to terrestrial viewing as Saturn’s.
To the naked eye, Saturn appears to be a bright star, and can be difficult to see. When viewed through a telescope, however, it becomes quite clear that it is a planet, with the rings plainly visible.
Saturn, like Jupiter, is always very visible in the sky. It shines brightly as a yellow star most all the time. It also takes comes into opposition about once a year for optimal viewing.
Uranus, Neptune & Pluto
The last three planets of our solar system, because they are so distant from us, are very difficult to see. It is almost impossible to distinguish them from other stars with the naked eye, and even binoculars do not help much. While easier to see with a telescope, they can still be difficult to locate.
If you are going to attempt to observe one of these planets, you are going to need to plan ahead. The other planets in the solar system can usually be seen, although for some it takes more looking than others when they are not in the right position. For these planets you will need to know exactly when and where to look.
Unfortunately you will not be able to see these planets in great detail, even with a telescope. However, they can still make for very interesting viewing.
All in all, watching the planets is a past time that can make you more aware of our little corner of the universe and bask in the glories of the celestial bodies. While most of the planets can be seen by the naked eye at most times of the year, planning ahead is always best.
Make sure you know when the best time to look is, and where in the sky you are going to be looking. Take some binoculars and a telescope to get the most of your viewing pleasure.
