29:51 Introductory
Astronomy Lab
Experiment 6
Week of April 23,
2001
Mars: The Angry Red
Planet
Of all the topics in solar system astronomy, probably none commands more interest among professional scientists as well as lay people as the geological and hydrological history of Mars. This topic is tied up with the question of life in the universe, in ways which were discussed in 29:52, Characteristics and Origins of the Solar System. In the lectures and textbook, there was discussion of features such as the Hebes chasma, the (possible) evidence for an ancient polar ocean, and the outflow channels and valley networks.
The purpose of this exercise will be to actually see the surface of Mars with your eyes. The Spring of 2001 is an ideal time to make these observations. Mars comes to opposition in late June, meaning for the next few months we will be unusually close to Mars. Telescopic observations will yield much more than is normally the case. For further information on this year’s apparition, check the May 2001 issue of Sky and Telescope magazine.
In many ways, the point to be made in this exercise is a negative one. Even at time of opposition, and in a good-sized telescope like the Celestron 8, observations at the eyepiece yield frustratingly little. This should impress two things on you. (1) The photographs we have of Mars (including photographs from the surface) and an amazing accomplishment of science and technology. (2) You can realize how primitive was our understanding of Mars before the space age.
The purpose of this exercise will be to observe Mars at the eyepiece of the Celestron 8, record any surface features that you can discern, then identify those features on a map of Mars. The two “prime dates” for carrying out these observations will be early on the morning of Thursday, April 26, or the morning of May 3. Some interesting or useful data on Mars for each of these days is given in the table below.
|
Quantity |
April 26 |
May 3 |
|
Right Ascension |
17h 50m |
17h 54m |
|
Declination |
-24d |
-24d 15m |
|
Distance from Earth |
0.691 au |
0.641 au |
|
Central Meridian Long. |
254 deg at 0h UT |
189 deg “ “ |
The parameter which has not been defined before is the “Central Meridian Longitude”. Astronomers have defined a system of latitude and longitude for Mars. The Central Meridian Longitude is the longitude that an Earthling observes to define the center of the disk of Mars. Since Mars rotates on its axis with almost the same period as Earth, the Central Meridian Longitude depends on the time of observation. The values in the table above are defined for 0hours UT.
Values for other times can be calculated as follows. Since Mars, like the Earth, rotates from west to east, an observer from deep space sees larger longitudes at later times. The longitude increases at a rate of 14.62 degrees per hour. We will make our observations around 5AM local time, which corresponds to 10 hours UT. For observations on April 26, therefore, the central meridian longitude at the time of observations will be
CML = 254 + 10 X 14.62 = 400d
Since 400 degrees corresponds to going completely around the globe of Mars (360 degrees) and then some, the longitude will be 400-360 = 40 degrees.
With this information and your drawing, we will still need a map. You can get yourself the May issue of Sky andTelescope, order Mars maps from Sky Publishing Corporations, or try the following web sites.
The first thing to do is to check out the position of Mars on your SC1 chart. This will permit you to learn something about the location of Mars in the sky, and learn some new constellations. Second, look over a Mars map and identify some of the features you might be able to observe. Note that the coordinates of “The Face” are longitude 40.8 degrees north, longitude 9.6 degrees. If these coordinates are on the disk, be sure to keep alert for signals beamed at us by the aliens.
Finally, during the observations use averted vision and wait for brief moments of extremely good seeing when you can observe subtle surface details. Make a sketch in your notebook of what you have been able to see. Later, identify the features and note your identifications in the notebook.