What can you tell about your location on the globe and the time of the year by observing the crescent moon?

At this time of year, the increasing amount of sunlight during the day and the later sunsets are noticeable to just about everyone. Also, most people probably recognize that the Sun's a lot higher in the sky at mid day now than it was a few months back. Obviously, the moon's position in the sky changes as well. Since we generally don't think that the full moon is much different from season to season, we may not be so observant in noticing that its position in the night sky varies considerably between winter and summer. When the moon is full, it's high in the sky during the winter and rides fairly low in the sky during the summer. When the Sun is high at mid day, the full moon at night will appear much lower in the night sky - the opposite is true during the winter months.

What's most noticeable about the moon is its changing shape. Of course, the phases that produce the changing shapes are governed by the moon's motion around the Earth. About every 29 1/2 days (synodic period), then moon completes one evolution around the Earth, and each day the amount of light reflected off of the moon, from our perspective on Earth, is slightly different. In a period of only two weeks, the moon can go from being completely illuminated (full moon) to being invisible to us (new moon). When half of the moon's disk is illuminated, the moon is said to be at quarter phase - it's 1/4 of the way around the Earth using the new moon phase as a starting point. This occurs about a week after the new moon and a week after the full moon, when the moon is 3/4 of the way around the Earth. In between the new moon and quarter moon phases, the moon appears as a crescent. In order for the moon to achieve the crescent shape, it must be closer to the Sun than is the Earth. The fatter the moon, the farther it is from the Sun.

As I write this, the moon is in the waning crescent - it's visible now only an hour or two before sunrise. On Monday, April 23, the moon will be "new" at 11:26 a.m. EDT. At this time, the moon is in conjunction with the Sun - it's roughly between the Earth and the Sun. If it were perfectly aligned with the Earth and Sun, a solar eclipse would be visible somewhere on Earth.

The plane of the moon's orbit is not on the same exact plane as the Earth's orbit, nor does it orbit directly over the Earth's equator. Rather, it's tipped about 5 degrees from the ecliptic, or the apparent path the Sun takes across the sky. Because the moon's orbital path changes throughout the course of the year, relative to the ecliptic, the appearance of the moon, in even the same phase, can be somewhat different from one season to the next.

For example, early next week, the moon will be a waxing crescent in the evening twilight - the illuminated portion will grow larger each evening. How will this crescent be different in appearance than during other times of the year? Only during the late winter and early spring does the crescent moon lies on its back, like a gondola, while during the rest of the year, it's like an archers's bow - the "horns" point to the left. This holds true for the middle latitudes only. If you live near the Equator, the waxing crescent moon is always on its back, and conversely, if you live near the north pole, the waxing crescent can only appear on its side. So, anytime you see just a thin slice of the moon in the early evening hours of February, March or April, you can expect to see that Cheshire cat grin in the gloaming.

If you want to see a grinning moon during other times of the year, you need to look in the pre dawn hours of August, September and October, when the moon is a waning crescent. Ok, you can see smiling waxing moons after sunset and smiling waning moons before sunrise at certain times of the year, but what you won't see is a whining or frowning moon anywhere in the world, except during the daylight hours. Even then, the moon is only in a bad moon around mid day, as if it's unhappy about having to be out in the bright Sun.

By the way, over the weekend, you may want to take a look to see if you can spot any "shooting stars" from the annual Lyrid meteor shower. It peaks late on Saturday and on early Sunday morning. The Lyrids typically don't produce that many meteors and the ones you can see aren't usually very vivid, but because there's no interference or competition from the moon, under clear dark skies, you might be able to see one every few minutes. The constellation Lyra will be visible in the northeastern sky after about midnight, but it's possible to see meteors before then, and you don't necessarily have to look toward Lyra to see them.