There are many good reasons to gaze at the moon, but this week will provide an extra-special one. On the night of July 27, some people will be able to watch the 21st century’s longest lunar eclipse, lasting 1 hour and 43 minutes.
What’s so special about a lunar eclipse?
A lunar eclipse isn’t the same spectacle as a solar eclipse: The darkness cast doesn’t have the same contrast. It results from a similar spatial arrangement as the other kind of eclipse: the Earth, sun, and moon aligning neatly.
In the case of a solar eclipse, the moon casts a shadow on the Earth. In a lunar eclipse, the roles reverse and the Earth casts a shadow on the moon.
The Earth typically experiences between one and four lunar eclipses a year. A fraction of those are total lunar eclipses. A tiny fraction still take place in July, when the Earth is as far as it can be from the sun, and thus able to cast the widest shadow on the moon. Even fewer lunar eclipses occur when the moon passes through the center of the Earth’s shadow, which increases the length of the lunar eclipse.
The July 27 eclipse will last for 1 hour and 43 minutes, which is very close to the theoretical maximum of 1 hour and 47 minutes. On Jan. 31 this year, the moon passed south of the center of the Earth’s shadow, so the length of the eclipse was only 1 hour and 2 minutes.
Where can I watch the lunar eclipse?
Those in the Eastern Hemisphere will enjoy the best of the lunar eclipse. Stargazers in Australia, Indonesia, and other eastern regions can see it as the moon sets, while those in Europe, western Africa, and South America, can see it when the moon rises.
The partial eclipse begins at 18:24 UTC (Universal Time), the total eclipse begins at 19:30 UTC, and the greatest eclipse will be at 20:22 UTC. You can find the exact time in your time zone by entering your location here.
Finally, you don’t need any special glasses to watch the lunar eclipse, because you won’t be exposing yourself to harmful direct radiation from the sun.
What’s about the blood moon?
Moonlight is nothing but sunlight reflected by the moon. When the Earth casts a shadow on the moon during an eclipse, it blocks out direct sunlight. But the moon still has a faint glow, usually an orange-red hue that many call the blood moon.
It occurs because of a phenomenon called the Rayleigh scattering, which is also what gives sunrise and sunset its orange-red glow. As sunlight passes through the Earth’s atmosphere, it hits different types of gases and particles. The process causes shorter wavelengths of light (blue and violet) to scatter more than longer wavelengths (red and orange). It’s the longer wavelengths that are able to reach the moon and give it the eerie glow.
If you were on the moon when the total eclipse occurs, you would see a red ring around the Earth. “In effect, you’ll be seeing all the sunrises and sunsets taking place at that specific moment on Earth,” according to Time and Date.
Correction: The article initially stated that red light has a shorter wavelength than blue light. It’s the other way around.