A lunar eclipse is when the Moon passes directly behind the Earth so that it is in the Earth’s shadow. Here is a look at how lunar eclipses work, how often they happen, and why a total lunar eclipse is called a blood moon.
- A lunar eclipse happens when the moon passes into Earth’s shadow.
- Lunar eclipses only happen when the moon is full.
- The Moon is full every month, but it does not get eclipsed every month because the Moon orbits the Earth at an angle, relative to the Sun.
When Is the Lunar Eclipse?
The only time a lunar eclipse occurs is during a full moon. But, not every full moon produces a lunar eclipse because the Moon’s orbit around the Earth is angled. A lunar eclipse only happens when the alignment between the Sun, Earth, and Moon is a straight enough line for the Earth’s shadow to block the sunlight.
How Often Do Lunar Eclipses Occur?
On average, about three lunar eclipses occur every year. Of these around 29% are total lunar eclipses.
How a Lunar Eclipse Works
The basic mechanics are simple. The Moon orbits the Earth and passes behind the planet into Earth’s shadow. This alignment of the Sun, Earth, and Moon is called syzygy. The shadow is dark, but the Moon doesn’t completely disappear because the Earth’s atmosphere refracts sunlight.
The two main types of lunar eclipses are partial lunar eclipses and total lunar eclipses. In a partial lunar eclipse, the Moon passes through part of Earth’s shadow. In a total lunar eclipse, the Moon fully enters Earth’s shadow.
The Earth’s shadow has two parts. The penumbra is the edge of the shadow, which is dim, but not completely dark because a little light scattered by Earth’s atmosphere still gets through. The umbra is the darkest part of the shadow.
In a penumbral eclipse, the Moon passes through Earth’s penumbral shadow, but not its umbral shadow. Most penumbral eclipses are partial eclipses that may not be very noticeable. Total penumbral eclipses are rare, but when the Earth’s penumbra entirely shadows the Moon, the Moon dims to a gray or golden color.
Some partial and nearly all total lunar eclipses are umbral eclipses. Here, the Moon travels into Earth’s umbral shadow. In order to get there, the Moon also travels through the penumbral shadow, so a penumbral eclipse can precede or follow an umbral eclipse. The Moon can turn a variety of colors, depending on Earth’s shadow and atmospheric conditions.
The duration of an eclipse varies according to how close the Moon and the Earth are to each other. The longest eclipses occur when the Moon is at apogee (furthest away), while the shortest eclipses occur at perigee (closest approach).
Lunar Eclipse vs Blood Moon
You may hear a total lunar eclipse called a blood moon. This is because the Moon sometimes appears deep red during a total lunar eclipse. But, not all total lunar eclipses are blood moons! The eclipsed moon can be red, orange, copper, or golden. The reason the Moon turns these colors is because violet, blue, and green light are more strongly scattered by the Earth’s atmosphere than yellow, orange, and red light. It’s the same reason sunsets appear orange and red.
Danjon Scale for Lunar Eclipses
Andre Danjon proposed the Danjon scale to describe the appearance of a lunar eclipse:
L = 0: Dark lunar eclipse where the Moon becomes nearly invisible at totality. When people imagine what a lunar eclipse looks like, this is probably what they envision.
L = 1: Dark eclipse in which the details of Moon are hard to distinguish and the Moon appears brown or gray at totality.
L = 2: Deep red or rusty eclipse at totality, with a dark central shadow but a bright outer edge. The Moon is relatively dark at totality, but easily visible.
L = 3: Brick red eclipse where the umbral shadow has a yellow or bright rim.
L = 4: Bright copper or orange lunar eclipse, with a blue umbral shadow and bright rim.
List of Total Lunar Eclipses
While lunar eclipses are fairly common, total eclipses are rare. On average, a total lunar eclipse happens once every 2.5 years. But, eclipses are not evenly spaced. For example, there are two total lunar eclipses in 2022, but the next one after that is not until the year 2025.
Here is a quick list of total lunar eclipses up to the year 2030, including the date and the time of greatest eclipse (UTC):
- May 16, 2022 (4:11)
- November 8, 2022 (10:59)
- March 14, 2025 (6:59)
- September 7, 2025 (18:12)
- March 3, 2026 (11:33)
- December 31, 2028 (16:52)
- June 26, 2029 (3:22)
- December 20, 2029 (22:42)
Can You Look at a Lunar Eclipse?
Viewing a lunar eclipse is perfectly safe. It’s easier on the eyes than looking at the “normal” full moon. In contrast, viewing a solar eclipse without eye protection can cause permanent eye damage.
How to Watch a Lunar Eclipse
Once you know the day of a lunar eclipse, you need to know whether or not it will be visible where you live. Look up “path of the lunar eclipse” online and verify you can see it. Everyone on the night side of Earth sees a lunar eclipse, weather permitting. A lucky few may see a selenelion, which occurs when both the sun and eclipsed moon are visible on opposite sides of the horizon. While a selenelion seems impossible, it occurs because the Earth’s atmosphere refracts light.
Lunar eclipses can last for several hours, but the duration of totality is much shorter, usually ranging from half an hour to two hours. During this time, just go outside and look toward the Moon. Remember, even though the Earth is between the Sun and the Moon, the Moon isn’t completely dark. It appears golden, orange, or red during totality.
References
- Karttunen, Hannu (2007). Fundamental Astronomy. Springer. ISBN 9783540341444.
- Liu, Bao-Lin (1992). Canon of Lunar Eclipses 1500 B.C.-A.D. 3000. Richmond VA: Willmann-Bell.
- Mucke, H.; Meeus, J. (1992). Canon of Lunar Eclipses -2002 to +2526 (3rd ed.). Astronomisches Büro Wien.
- Nigro, Nicholas (2010). Knack Night Sky: Decoding the Solar System, From Constellations to Black Holes. Globe Pequot. ISBN 978-0-7627-6604-8.