Eclipse! That’s the next cool astronomical event for us, and the lunar eclipse happens next month. The bad news: It starts after midnight and peaks about 4 a.m. The good news: It happens in the early morning hours of April 15, so many of us can walk outside, bleary-eyed from working on the taxes due that day, to enjoy the event.
The moon will begin edging into the Earth’s shadow just before 1 a.m. but will not enter the darkest part of the shadow for another hour. As it enters the darker “umbra” portion of the shadow, the shadow’s curved edge reveals the spherical shape of Earth, which cast it.
The lunar eclipse was an early proof that Earth was not flat, since the only shape that always casts a circular shadow is a sphere. Early Greek astronomers recognized this more than 2millennia ago and even used the relative sizes of the shadow and moon to estimate the size of the moon compared to Earth.
Note that eclipses are different from the crescent phases of the moon. Those are caused by its presenting different amounts of its night side to our view as it orbits the Earth. The shadow of the Earth is not involved in the phases of the moon.
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During a total eclipse, the color of the moon can range from dark gray to a ruddy red, depending on the cloud conditions around the Earth’s twilight zone at the time. If that band of longitude is cloudy, then little light filters through it to give the moon any illumination. If it is mostly clear, then we get a red eclipse from light that goes through the twilight band.
Why red? For the same reason the sky is blue. Sunlight is a spectrum of colors from violet to red, peaking in yellow-green. Sunlight is scattered by a phenomenon that preferentially scatters blue light more than red. The blue light is scattered in all directions, while the red portion continues on to the moon during an eclipse.
During the day, when you look up in the air, you see this scattered light coming from all directions – the blue sky. At sunrise or sunset, the sun is orange or red because that blue light was removed from the beam on the way to your eyes.
If bad weather steals our view, we will have the next chance on Oct. 8. That event, again inconveniently during the business/school week, is less favorable. The umbral phase will begin after the start of dawn, when the moon is about to set and is low in the western sky.
Let’s hope for clear skies in April!