Have you ever walked in to a Foreign language class that only spoke the language being taught? I had the misfortune of doing that in College. I had no idea what anyone was saying and it was very confusing for me. After a week of that assault, I dropped the class. It was not profiting me in any way and was way over my head. That's a whole lot like astronomy in many ways. I was fourteen at the time and I had found this Astronomy club to go to. I had been in the astronomy game for half of my young life so I knew my way around the sky pretty well. I came in to the club meeting and sat down to listen in on the goings on of this club. I remember a giant argument ensuing over proper motion or something. I am judging by the looks around the room it was over the heads of 95 % of the people sitting there. I left and called my parents for a pickup. I made a promise to my self that night that I would try to make astronomy easy for people to understand. So with that in mind...I am going to go over some words commonly used in astronomy and explain them so you can get a clear understanding of the word and it meaning. Right now, I am working on a primer for new amateur astronomers. For new astronomers entering the game we love so well it is a difficult thing to grasp if you have no frame work from which to build your knowledge. So let's take a look at where we live and learn a few words that will make us look good when we throw them out there in a conversation with your fellow starry eyed friends.
Every thing in the Solar system orbits (circles) around the Sun. You might say now hold on! The Moon orbits around the Earth and I would say that is correct but Mother Earth drags the Moon right with her as she makes her way around the Sun. In the bigger picture the Moon does orbit the Sun also. This truth can be applied to all the planets as well as comets asteroids. So Now you have an idea of what is going on in the Solar system. Let's talk about the orbits of these planets for a minute.
The orbits of the planets are not circular They are really ellipsoidal ( a stretched out circle) and the ellipse is not centered around the Sun. That means sometime the Planet is closer to the Sun than other times. For a planet, the closest approach to the Sun on it's orbit is called Perihelion and of course if there is a closest, there must be a farthest point of the orbit also and that is known as Aphelion. That's pretty easy to follow for the most part. Each planet has their own aphelion and perihelion but when we are observing them there are other terms that pop up.
For the inner planets from Earth, Venus and Mercury never seem to too far away from the horizon. From Earth's vantage point, the farthest away from the Sun mercury gets is 28.7 degrees. Venus is 47 degrees. Earth's orbit and inner planets has to be just in the right spot and that spot for Mercury or Venus to briefly hang is called Greatest elongation. Since Mercury is orbiting the Sun in 88 days and Earth orbits in 365, the angle of separation for viewing mercury changes quickly. In other words sometimes Mercury is in front of the sun or behind it in relationship to earth. In either case, we can not see the fleet afoot Mercury. These spots in the orbit where this happens when observing from Earth are called Superior conjunction and inferior conjunction (inferior means Mercury or Venus is between the earth and the Sun while Superior means the Sun is directly between the Earth and either Mercury or Venus) Another interesting feature of Venus and Mercury are that they have phases like the Moon. (crescent, quarter, gibbous and full. Astronomers will give a percentage of the disk illuminated for these two.
The Outer planets(Superior planets), Mars and beyond play a little different. These planets have no phases. They show a full disk at all times towards Earth. Alas there are times when you can't see them due to their position in relationship with earth and the Sun (sound familiar?) Sometimes the planet is directly behind the sun and that is definitely a no see (They are in Conjunction with the Sun) but after it clears the Sun's disk it could be seen if it were not daylight. When it is dark and we are looking away from the sun any planets that are on the same side of the solar system as earth can be seen at night! When the orbits are as close as they are going to get for the year the two planets are said to be in Opposition. Mars will be in Opposition to Earth on Jan 29th, 2010.It is a great time to get to a Telescope and take a look. Saturn will be in Opposition on March21st, 2010.
A curious thing happens at Opposition the Planet appears to start working its way to the east then it starts to move west again. This odd happening is called Retrograde motion. Most of the time The outer planets seem to orbit just fine in Regular motion. The appearance of going backward is caused by The earth passing the outer planet and by doing this, gives the illusion of the reverse course. So now we know where we are in the solar system compared to the other planets and how it works generally. That is a frame work. Next week we talk about distance...
Clear skies and great seeing too!