by James Morris
Last night, many of us viewed a rare spectacle – a total eclipse of a “supermoon.” And this morning, Facebook was abuzz as we woke up to friends and family sharing photos of the fully eclipsed, blood-red moon.
From the time I was 12, I have been trying to capture something a little different – the moon slowly going into eclipse, getting smaller and smaller, dimmer and dimmer.
Last night gave me one more try:
This photo not only gives a sense of the moon slipping into darkness, but also invites us to consider what’s going on. If the moon is moving into the Earth’s shadow, why does it rise up and to the right, but get eclipsed on the left?
The rising (and setting) of the moon simply has to do with the rotation of the Earth. As the Earth rotates, it appears as though the moon is rising and setting, but that’s just because it is relatively still while we are spinning around the Earth’s axis. It’s the same reason why the sun rises and sets. And it’s the same reason why the stars and planets, if you watch them throughout the night, also rise and set. That’s all the rotation of the Earth. And it really has nothing to do with the lunar eclipse.
An eclipse of the moon occurs when the shadow of the Earth falls across the moon. The Earth, lit by the sun, is always half lit (the day side of the Earth). It therefore always casts a long cylindrical shadow behind it, coursing through space.
The moon rotates around the Earth, taking about a month to complete a full circle. When the moon is full, it is on the opposite side of the Earth from the sun, so that, from our vantage point, the fully lit side is facing us. Therefore, every month, it has a chance to enter the Earth’s shadow, but most of time it misses, passing over or under the shadow. During a lunar eclipse, however, the moon enters that shadow, and we witness a lunar eclipse, like we did last night.
So, the reason why the left side of the moon falls dark first is because that’s the side of the moon that is entering the Earth’s shadow. We have a chance to see, in real time, the slow, steady movement of the moon around the Earth.
In this photograph, taken above Brookline reservoir in December 1992, we are essentially seeing two motions at once – the rising of the moon due to the rotation of the Earth, and the eclipsing of the moon due to the movement of the moon into that long, dark shadow of the Earth:
Capturing these two movements on film (or, these days, in pixels) isn’t easy. In my “ideal” photograph, the moon begins full in the bottom left, and is fully eclipsed in the top right. This is what I tried to get in this photograph, taken in Harvard, Massachusetts in August 1989, except that the moon was covered by clouds at the start of the eclipse:
That means, I need to figure out how much time it will take the moon to pass across my viewfinder. If it takes about an hour for the moon to go from full to eclipsed, then I want to zoom in just enough so that the moon makes it from corner to corner in an hour.
Luckily, I don’t need to wait for an eclipse to figure that out – I can do that any night the moon is out, since the motion of the moon across the viewfinder (and across the nighttime sky) is simply due to the rotation of the Earth about its axis.
Exposure is a bit tricky too. If you set your camera to automatic, it tends to overexpose the moon, like in this photo of the Harvest moon above the Boston skyline in October 1990, where the moon appears as a white circle, and we miss out on seeing the craters and plains (called maria) we can see when we look closely, the “face” of the man in the moon.
You can look up moon exposures on the internet, of course, and it’s important to keep in mind that exposure changes as the light of the moon is dimmed.
Again, this is something that you can practice by taking photos of different phases of the moon. The phases of the moon look kind of similar to the moon as it goes into eclipse, but the two have completely different explanations. When the moon is partially eclipsed, and therefore looks like a quarter or crescent moon, we are seeing the shadow of the Earth on the moon. We can imagine that if someone were standing on the “side” of the Earth making hand shadows, we would see them projected on the lit face of the moon.
But a quarter or crescent moon is not in the Earth’s shadow at all. It only appears to us as a quarter or crescent moon because of our position on Earth relative to the lit side of the moon. When the lit side faces us, the moon is full; when the lit side is away from us, the moon is new; and when the lit side is to the right or left, we see a quarter moon. But, in all of these phases, the moon is half lit, just like the Earth, since it is a sphere in space lit up by the sun.
That’s true even in this photograph of the crescent moon over the Charles River at sunset:
The inspiration for the photograph I took came from my father, who did similar lunar research and captured this one from the rooftop of his apartment in Brooklyn, NY in the late 1940s or early 1950s:
© James Morris and Science Whys, 2015