What is a lunar eclipse, and why does it turn the Moon red?

A lunar eclipse occurs when the moon passes into the shadow of the sun. This happens when the sun, Earth and moon line up, with Earth in the middle. In a total lunar eclipse, the moon passes completely into the central portion of Earth’s shadow. If you were on the near-side of the moon during a total lunar eclipse, the entire surface of the sun would be blocked by Earth’s surface.

During a total lunar eclipse, the only light that reaches the surface of the moon is refracted through Earth’s atmosphere, which essentially acts like a lens. Light is a wave, and every color of the rainbow has a different wavelength — red the longest and violet the shortest. As it happens, short-wavelength light is much more likely to be scattered when it passes through the atmosphere. This is why the sky looks blue during the day — the blue and violet sunlight passing overhead is scattered off its path, and some of it reaches our eyes. The red light isn’t scattered nearly as much, so more of it can pass through the atmosphere and, during a total lunar eclipse, reach the moon's surface.

Unlike during a solar eclipse, no special safety precautions are necessary — it's perfectly safe to look at the moon (in or out of a lunar eclipse) with the naked eye. A pair of even low-power binoculars will certainly help show the moon in more detail. For photographing the moon, I would recommend a camera with some optical zoom — the moon is only about 0.5 degrees across. Because of the low lighting conditions, a tripod or other support would be helpful since a longer exposure time will be needed.

On average, there are about two lunar eclipses per year, but only about one third of lunar eclipses are total. The next lunar eclipse will be in August; it will be a partial lunar eclipse. The next total lunar eclipse will occur at the end of 2028.

One of my favorite things about total lunar eclipses is that it’s hard to know ahead of time quite what it will look like. The moon can take on a color ranging from burnt orange to red to grayish brown. The closer the moon passes to the center of Earth's shadow, the darker the color will be. Also, since the sunlight that reaches the moon during a total solar eclipse first passes through Earth’s atmosphere, atmospheric conditions — including clouds and even volcanic activity — can significantly affect the moon’s color.