Last night's "blood moon" was the first of four total lunar eclipses that will occur in a regular pattern over the next two years. This series of four is known as a "tetrad," and it's caused by the shape of Earth's orbit.
The first eclipse of the current tetrad came in the pre-dawn hours of April 15th, when the full Moon passed through Earth's shadow. The next total lunar eclipse comes Oct. 8, 2014, then April 4, 2015, and the last on Sept. 28, 2015. Eclipses can be seen from only part of the Earth, but coincidentally, all four of this lunar tetrad will be at least partially visible from North America.
"During the 21st century, there are 8 sets of tetrads, so I would describe tetrads as a frequent occurrence in the current pattern of lunar eclipses," said NASA eclipse expert Fred Espenak in a press release. "But this has not always been the case. During the three hundred year interval from 1600 to 1900, for instance, there were no tetrads at all."
Astronomer Giovanni Schiaparelli noticed in the 19th century that there are periods when tetrads occur comparatively frequently, interrupted by eras when they are rare. More recently, astronomers have calculated that tetrads peak every 565 years. For example, there were no tetrads from 1582 to 1908, but 17 tetrads between 1909 and 2156. The most recent tetrad fell in 2003-2004.
The 5000-year period from 1999 BC to 3000 AD contains 142 tetrads, accounting for about 16 percent of the 3479 total lunar eclipses during that period. During that same 5000-year window, there were 4378 penumbral eclipses and 4207 partial lunar eclipses.
The tetrad "seasons" are tied to the slowly decreasing eccentricity of Earth's orbit, which is still slightly oval-shaped. Once Earth's orbit becomes a perfect circle, in the distant future, tetrads will no longer be possible.
In general, solar and lunar eclipses occur about 3 to 5 times every year. Most years see two lunar eclipses, but it is rare for both to be total.
There are three types of lunar eclipses: Penumbral, partial, and total. All are caused by the alignment of the Earth, moon, and sun. When the three are perfectly aligned, you get either a total lunar eclipse, like Monday night's, or a total solar eclipse.
During a total lunar eclipse, the Earth is between the moon and sun and blocks all direct light from the sun. A small amount of light travels through Earth's atmosphere and is bent toward the moon, staining the moon with a sunset-red glow.
During a partial lunar eclipse, the Earth blocks a chunk of sunlight, throwing a visible shadow across the moon's surface. During a penumbral eclipse, the moon passes through the outermost section of Earth's shadow and looks dusky or smudged. It's such a subtle effect that, even if you're looking for it, it's hard to notice.
The next total lunar eclipse, on Oct. 8, will be visible in its entirety for all of Europe and Africa. People in South America and the eastern half of North America will be able to see the first part of the eclipse, while stargazers in Asia and Australia can catch the tail end of the eclipse.
If you don't want to wait until October, there is another eclipse in just two weeks – an annular solar eclipse, which shows the sun's glowing rim encircling the moon – but you'll have to travel to Antarctica, which is the only part of the Earth from which it will be visible.
You'll have better luck with the partial solar eclipse on Oct. 23, 2014, which will be visible (weather permitting) in Canada and every state except Hawaii.