Big and Bright Moon
Published on Jan 10, 2009 at 11:19 am.
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Filed under astronomy, moon, skywatching.
Tonight is Full Moon. Actually, it occurs at a little before 9:30 tonight, Central Time, in the USA. It is very early Sunday morning in Europe. Full Moons occur about every 29.5 days. So, you get one every month. Every few years, you can get two in a month (one at the beginning and one at the end). In North America, we often call that a “blue moon.” I wrote about that a couple of years ago. But, that’s obviously not the case tonight. Still, many Full Moon’s are given names under tradition. Tonight’s Full Moon is called the Wolf Moon.
But, that’s not all that is special about tonight’s moon. The Moon is in a slightly elliptical orbit around the Earth. In ranges from about 364,400 kilometers to about 406,700 kilometers from Earth. These numbers vary a bit due to gravitational influences of other bodies in the Solar System (the Sun and planets). The closest point that the Moon gets to Earth is called perigee, and the farthest is apogee. The Moon was just at perigee. At 4:53am, CST, it was at an unusually close perigee of a bit under 358,000 kilometers. In fact, that is about the closest that it will be all year. Since the synodic period of the Moon (the length of a cycle of phases) is longer than the orbital period, this will be the closest Full Moon of the year 2009. When the Moon is closer, it obviously looks bigger. So, that will make the Moon look particularly large tonight. But, really the Moon’s orbit is not so extreme that a casual observer would notice. Tonight’s Wolf Moon will only be about 14% larger diameter than the smallest Full Moon of the year. You might notice that, but only if you were to try to measure it. To the naked eye, it would be hard to tell the difference unless the small moon and large moon were placed side by side in the sky to compare.
But, there is something else interesting here. The Moon shines by reflected sunlight. The closer it is, the more area of the sky that it covers, and so the total amount of light shining on Earth increases. That makes closer Full Moons brighter than their more distant sisters. Oh, but that is not all. The brighter the Sun appears to be shining, the brighter the Moon will appear to be. But, the Sun is pretty constant, right? Well, yes and no.
The Sun does undergo minor fluctuations in energy output. But a much bigger factor is that Earth’s orbit, itself, is a bit elliptical. Earth was closest to the Sun (called perihelion) on January 3 (or January 2, here in North America). We are still pretty close to that date, so the Earth is still nearly as close to the Sun as it will get. Since the Moon orbits Earth, that makes the Moon, also, about as close to the Sun as it gets. Being near perihelion makes the Sun appear brighter. Since the Moon shines by reflected sunlight, it will look brighter, too. Putting all this together, the Moon will appear nearly 1/3 brighter than normal tonight.
Now, there is one final bit of astronomical aligning going on here to make the Moon even more prominent to observers in the Northern hemisphere. Since the Full Moon appears opposite the Sun in the sky (which is why it is full in the first place), then we need to look at where the Sun appears in the sky. As the Earth moves about the Sun, its axis of rotation is tilted about 23.5 degrees from the axis of its orbital motion. This is what gives the Earth its seasons. Back a few weeks ago, on December 21, Earth was oriented so that the Southern Hemisphere was tilted the most towards the Sun and the Northern Hemisphere was tilted the most away from the Sun. That was the Winter Solstice. It makes the Sun appear to be the farthest south in the sky that it can be. For observers in the mid-northern latitudes, this means that the Sun rises in the southeast, is fairly low in the southern sky at midday, and sets in the southwest. Since, we are only a few weeks from the solstice, the Sun is nearly as far south as it was then. But, remember, the Full Moon is opposite the Sun in the sky. That puts the Full Moon during this time of year nearly as far north as it can go. That means that it rises in the northeast, rises to be very high in the sky (almost overhead, here in Texas) in the middle of the night, and sets in the northwest. The higher an astronomical body is in the sky, the brighter it appears because the light does not have to pass through as much atmosphere. That means that winter Full Moons will always look brighter than summer Full Moons (since in the summer, the Sun is highest in the sky, so the Full Moon is lowest).
So, putting all this together means that tonight’s Full Moon, for observers anywhere, but especially for those in the northern hemisphere, will be the biggest and brightest Full Moon of 2009. That is why I took a break from blogging about the microgravity flight in order to post about the Moon today.
-Astroprof
Moon photograph taken by Astroprof
