The best time to observe the Moon
Published on May 9, 2006 at 6:43 pm.
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Filed under moon, skywatching.
When is the best time to observe the Moon? People often ask me this. They assume that the best time will be full moon. Well, full moon is coming in a few days. So, that’s the best time to set up your telescope and look, right? Well, probably not. Really, it depends upon what you want to observe. The Moon is pretty complex, so the best time to observe features might not all come at the same time. In fact, what I like to see are the craters and mountain ranges. Actually, the two are related. The Moon does not have plate tectonics like Earth, and no tall volcanic mountains. The mountain ranges on the Moon are really the walls of giant impact basins. The mountain ranges and the crater walls are similar in color to their surroundings, so there isn’t much contrast. That means that they are tough to make out, sometimes. However, when the Sun is hitting them at a low angle, then they cast long shadows on one side. The contrast between the black shadow and the brighter material makes the craters and mountains stand out. You can often see features that you could not otherwise see very easily. At full moon, the sun appears to be shining straight down on the Moon, as seen from Earth, so there are no shadows. However, at the quarter moon phases, then the Sun shines from the left or right, and so sunrise or sunset on the Moon runs right down the middle of it, as seen from Earth. That makes features near the middle of the Moon stand out. A crescent moon makes features to the left or right of the mid line stand out better (depending upon whether it is a waning crescent or a waxing crescent). Theoretically, a similar effect should happen at gibbous phases, but personally I think the crescent moons show better relief. What is particularly cool is when the terminator (the sunrise/sunset line) runs right through a mountain range. Then, you can see occasional peaks of mountains on the dark side of the line sticking up into the light. They look like little white dots just to the dark side of the Moon. Really cool.
So, does that mean that the full moon is worthless for observing? No! As it turns out, if you are looking to observe the seas, then the full moon is excellent. The Moon doesn’t have any water, of course. The things that we call seas are really flood basalts that filled some of the largest impact basins and sometimes (as inthe case of the Ocean of Storms) some of the surrounding land. These flood basalts are darker in color than the surrounding highlands, so they do stand out even during full moon. The basalts are also ever so slightly bluer than the highland rocks. So, I often use a red filter to observe the full moon. This has a double benefit of both improving contrast between the seas and the highlands, but also it dims the Moon. The full moon is annoyingly bright when viewed through an unfiltered telescope. The red filter is easy on the eyes. Interestingly, I also find that a yellow filter tends to show off some of the rays of the craters better.
Speaking of bright moons, the full moon is so bright that you can read by the light of it outdoors at night. City people don’t always believe me when I say that, but it is true! Let yourself get slightly dark adapted, and you can read by the light of the Moon! It is amazingly bright out at night during full moon. Go out this weekend and see for yourself. Even in the city, with all the light pollution, it is bright.
-Astroprof
