“The moon is eclipsed through the interposition of the earth… Anaxagoras was the first to set out distinctly the facts about eclipses and illuminations.”
–Euripides, in Hippolytus, 431 B.C.
As we all know, the Earth revolves around the Sun while it rotates on its axis. The rotation causes day-and-night every 24 hours, while the revolution causes our seasons and our calendar year.
You’ll notice, in the image above, that the Summer Solstice — June 21st — is when the North Pole of the Earth is tilted most directly towards the Sun, and that the Winter Solstice — December 21st — is when it’s tilted most directly away from the Sun.
But you’ll also notice something else: the Earth moves around the Sun in an ellipse, not a perfect circle. And the Earth gets closest to the Sun just slightly after the winter solstice, in early January, and gets farthest from the Sun in early July. This makes for a number of interesting things.
But these two phenomena — the non-circularity (eccentricity) of the Earth’s orbit and it’s tilt (obliquity) — make for a fun project. If you take a picture of the Sun at the same time every day, what would you see?
You’d get a shape known as an analemma, which is a combination of the effects of our elliptical motion around the Sun and our tilt, as we rotate, on our own axis. The point at the very top corresponds to the Summer Solstice, when the Sun is highest in the sky, and the point at the bottom is the Winter Solstice, coming to you in a mere four days.
In the meantime, the Moon orbits the Earth. Once every four weeks or so, the Moon makes its own ellipse around the Earth. When the Moon is roughly between the Earth and the Sun (on the “near” side), we get a New Moon, and when it’s on the opposite side of the Earth from the Sun (on the “far” side), we get a Full Moon. But you might expect, if the Earth is between the Moon and Sun, that’s the Earth’s shadow would block out the Moon.
And yet, we haven’t had a Total Lunar Eclipse since February of 2008! Why not?
Because the Moon doesn’t orbit in the same plane that the Earth orbits the Sun! The Moon’s orbit is tilted, and it’s only on those fortuitous occasions — where the Full Moon passes through the plane of Earth’s orbit — that we get a lunar eclipse.
The Earth is always casting a shadow, thanks to the Sun, but it’s very rare that the Moon even partially passes through it; it normally happens just once or twice a year.
But this year is extra special. You see, we’re not only getting a Total Lunar Eclipse this year.
We’re getting it on the Solstice! Starting at 10:32 PM, Pacific Standard Time (it’s where I live) on the night of December 20th, the Moon will start to slip into the Earth’s shadow, and will start to be blocked out. 68 minutes later, the eclipse will be total, and will remain so for 73 minutes, well into the early morning of December 21st. What will the Moon look like, with respect to Earth’s shadow?
Pretty amazing. So go out and have a look at this once-in-a-lifetime treat on the longest night of the year! Where can you see it from?
It’s best in North America, but look for it in Europe near Moonset and in Japan near Moonrise, too!
The last time we had a Total Lunar Eclipse on the Winter Solstice? 1554. No kidding. So enjoy an eclipse on the longest night of the year, for what’s likely to be the only opportunity you’ll ever have! Me? I’ll be hoping for clear skies. It could happen…