Even professional astronomers,
after long nights observing distant galaxies, quasars, and pulsars will
occasionally turn their gaze upon our sister world, the Moon.
From a dark sky, on a clear night, the Moon can even produce shadows here on Earth. Every now and again, we too, gaze upon our lone sister world. We have looked at binocular highlights, and this week, we shall discuss some beautiful areas of the Moon which a telescope, even a modest backyard telescope, may reveal.
As with most observing sessions, remember it is best if you start with a very low power eyepiece (often having a focal length of 20 or 25mm), and work upward only as the evening allows. Remember, the longer the focal length of the eyepiece (which is usually the only number written on an eyepiece), the lower your magnification. For instance, a 10mm eyepiece delivers twice the magnification of a 20mm eyepiece. The cost of magnification, however, is a loss of your field of view (how much can be seen at one time), and a loss of image brightness. In fact, both of these go down as a square of the increase in magnification.
For instance, in the example above, when we changed from a 20mm to a 10mm eyepiece, we doubled our magnification. However, we are now only receiving 1/4 of the amount of light we were before, and we are also only seeing 1/4 of the amount of area we were seeing previously. In the case of the Moon, the loss of image brightness presents no problem. In fact, the loss of light can be a benefit in observations of the Moon; she is so bright, more detail can be seen if less light is allowed through the telescope.
My favorite nights of lunar observing often involve viewing the Moon at at least three different powers, and different aspects of the Moon stand out in each. Therefore, that is how we will discuss telescope observations of the Moon.
The first choice in an evening of lunar observing is to choose a promising night. The best evening views of the Moon can be seen when the Moon is in it's first quarter stage. At this point, the Moon will be nearly due south at sunset, and will set at midnight standard time. The best predawn viewing is when the Moon is in the third quarter stage. At this time, the Moon rises at midnight, and is due south at dawn. When the Moon is full, rising at sunset, and setting at dawn, presents a poor time to observe our lone satellite. The Sun is shining directly into the craters at this time, producing little to no shadow.
Go outside with a moon map, and a flashlight with red cellophane taped around the lens. This will allow you to see the map, and still retain your night vision. First starting with your lowest power eyepiece, usually having a focal length of 20-25mm, turn your sight to the moon.
Most backyard telescopes will reveal the entire Moon with an eyepiece in this range. Often, this will produce magnifications of 40-60X. Your image will likely be reversed left and right, but if you are using a star diagonal (a right angle bracket with a mirror between your telescope and eyepiece), your image should be correct up and down.
Observe the mountain ranges, and plains, stretching for thousands of miles across our sister world. Look how the terrain changes over various parts of the lunar landscape. Next, change to a medium power eyepiece, maybe around 12.5mm. Now, we begin to observe shadows cast by the mountains of the Moon. Also, we begin to perceive detail in how one area of the Moon changes to another. The erratic transition, say, from plain to mountain ranges.
Lastly, finish with a high power eyepiece. Something in the range of 6mm. Now detail in the craters can be seen, and rubble spread for miles away from where a meteor hit long ago. On a clear night, the lines created by shadows from crater walls dances with the crater itself. At all powers, try to find objects which are listed on the map. Can you find, for instance, the crater Tycho? Observe the mountains in the southern highlands; this is my favorite area of the Moon. For fun, try to observe Mare Tranquiliaris, where Apollo 11 landed. Remember, sometimes, the most beautiful places are right near your home.
Clear skies, and good viewing.
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