Sunday, January 4, 2015

Why Is The Full Moon Higher In Winter Than In Summer?

Tonight, January 4th, the moon is full. Yea! 

At 8:53 p.m. on the West Coast, to be exact. (On calendars, it says the full moon is January 5th, but that's because the official date of the full moon is based on the time in Greenwich Mean Time, otherwise known as UTC - Coordinated Universal Time.) And when it is 8:53 p.m. on the West Coast, it is 4:53 a.m. the next day in London. Which explains why we often look up at the moon on the "calendar date" of the full moon and think, "I think it was fuller last night." Because, for us, the full moon was the night before.

In Tahoe, we often notice the full moon because:
1) Our nights have less light pollution, so the moon is brighter
2) We're at high altitude, so there is less atmosphere above us to dim the moon
3) The snow on the mountains is spectacular in the moonlight, begging us to notice

This is from a cool website called

If you notice full moons, you can't help wondering why the full moons of winter are way, way up high in the sky, making it so bright on the snow that you can easily read by moonlight. By comparison, summer full moons are low in the sky.

Ever notice how the full moon track is just like the sun's track, only the summer and winter tracks are reversed?

I wondered why that is, so I looked it up.

It turns out that it has to do with the fact that the Earth is tilted on its axis (which is what gives us our seasons among other cool stuff). And when the moon revolves around the Earth, it doesn't stay over the Earth's equator, it stays on the same plane as the Earth's orbit and the sun and most of the planets. 

That plane is called the ecliptic, and all the stuff that stays on the ecliptic plane got that way because when the solar system first began to coalesce, the material that eventually became the planets contracted into a spinning disc. The spin and plane of that disc has stayed the same ever since.

Here's an easy way to visualize why the full moons are higher in winter than summer. 

First, remember that when the Moon is full, it is on the opposite side of the Earth from the sun, so its entire surface that we see is illuminated.

Now think of a globe sitting on a really large table. The table represents the ecliptic, and most of the stuff in the solar system stays on that table. As you know, a globe accurately depicts the Earth at a tilt. The Earth, like the most of the solar system's inhabitants, also stays on the ecliptic plane as it orbits the sun. It just stays tilted as it moves around. 

When the Earth's North Pole is pointing toward the sun, it makes for the long days of summer for those of us in the northern hemisphere. But when the Earth rotates so that we are in the dark of night and we look up to see the full moon, it is on the opposite side of the Earth from the sun. So when the sun is high in the summer, that means that the moon is low.

In winter, the opposite is true. The Earth's North Pole points away from the sun, so the sun is low in the sky. The full moon, on the opposite side of the Earth from the sun, is now high in the sky.

So enjoy the glorious full moon tonight. Along with the full moon we had in December, tonight's full moon will be higher in the sky than any other until next December. And with the snow on the mountains, Tahoe's full moon is a real treat!

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