A Fortnight In A Day

We are all such creatures of the indoors that there are some simple and obvious things about the sky above us that we don’t really get. Most Americans would not know the phase of the Moon unless they were looking at it, and a lot of folks don’t realize you can see the moon in daylight just fine (let alone Venus, under the right conditions). GG likes poking around in such little oddities as the offset from the earliest sunset to the shortest day. Here’s another one: you can kind of see the whole year of sunshine play out in a month of watching the Moon.

If you look for the Moon at moonrise on successive days, you will notice it moves around. A lot. As the Moon is pretty close to the ecliptic, you are in fact seeing the same motion that sunrise makes over the course of a year, though the phase varies with the season. Right now (Nov. 25th) the Moon rose nearly at the same place the Sun will rise on the summer solstice–the longest day of the year. In a couple of weeks, it will rise near the place the sun will rise a few days later, on the winter solstice. Of course, you might not notice that as it will only be a narrow waxing crescent just two days after being new.

How long can you see the Moon on a given day? Most people might think 12 hours (maybe even less). If you recognize that the lunar month means that the Moon has to make a circuit of the sky in about 28 days, you might guess 12 and a half hours–and on average you’d be close (if you guessed 11.5 hours, you just had the Moon in a retrograde orbit). But here is the difference between moonrise and moonset for Denver this November:

LunarDay

The average is a bit less than 12 1/2 hours (about 12:24 here), but you can see that some days you can see the Moon a long time, and other days you don’t see it for long at all.  Near the winter solstice (either hemisphere), the full moon is high in the sky a long time (full moon above is on the 22nd)–just like the Sun near the summer solstice. But in the summer, the full moon is low in the sky and not up nearly as long.

This can drive photographers batty. First, the Moon will rise in a different spot nearly every night–the exceptions are when it is near the northernmost or southernmost positions. The second is that the timing of moonrise will vary–and not just by that 48 minute average difference between a full lunar “day” and a solar day. When the lunar day lengthens the most, the moonrise will come only about 30 minutes later each night–around the 18th day in the plot above. And then when the lunar day shortens the most, as near day 3 above, it will take over an hour. These extremes are when the north-south position of moonrise changes the most.

All this is because the Moon traverses the ecliptic in just under a month while the Sun takes a whole year on the same journey. One night for the Moon is 12 days for the Sun. So when it seem like it will take forever to get through winter, go look for the moonrise and see it change day to day. It might help the long winter nights pass more quickly…

One has to wonder how often archeologists look for lunar alignments in ancient constructions.  There is often considerable attention paid to solar alignments, and for good reason, but don’t you think that sometimes ancient peoples might have marked lunar positions as well?

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One response to “A Fortnight In A Day”

  1. Paul Braterman says :

    Thanks. I’d never realised!

    Like

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