A selenelion is a special lunar eclipse where both the Sun and eclipsed moon are visible in the sky at the same time. Another name for this phenomenon is a horizontal eclipse. During a selenelion, you see the eclipsed moon on the horizon and the rising or setting sun opposite it.
How a Selenelion Eclipse Works
A selenelion seems impossible. How can you see the Sun and the Moon at the same time when they have to be on exactly opposite sides of the Earth during a lunar eclipse? The answer is that atmospheric refraction makes both the Sun and the Moon appear higher in the sky than they really are, geometrically speaking. You always see them rise a bit sooner (about 0.5°) than they actually clear the horizon. Similarly, you see the Sun and Moon in the sky a bit longer than after they set.
While a selenelion total eclipse (blood moon selenelion) is the most spectacular form of the phenomenon, selenelion partial lunar eclipses also occur. However, because the horizon is relatively bright, these partial lunar eclipses are not very visible.
A selenehelion works the same way. The difference is that the eclipsed Sun is visible on the horizon, with the Moon opposite of it.
Is a Selenelion Rare?
A selenelion is not rare. It occurs somewhere on Earth for every lunar eclipse. The trick is choosing the right location, where either the Sun is rising and the eclipsed Moon is setting or else the Moon is rising and the Sun is setting. Sometimes this is out in the middle of an ocean, making viewing inconvenient for most people.
So, if a lunar eclipse occurs on average three times a year, you have around three chances per year of catching a selenelion. Only around 29% of lunar eclipses are total lunar eclipses, so a selenelion total lunar eclipse is relatively rare.
How to See a Selenelion Eclipse
Now that you know what a selenelion is, maybe you want to see one for yourself. The first step is looking up the dates of upcoming lunar eclipses. Next, Google the eclipse path so you know where it’s visible. The selenelion is visible where the eclipse is visible at moonrise or moonset. Because the Sun and Moon are opposite one another during a lunar eclipse, an eclipse at moonrise also occurs at sunset and an eclipse at moonset also occurs at sunrise. (The exception is near the poles because the Earth is also tilted on its axis.) Usually, when a selenelion is visible over populated areas, it makes the news, so you know exactly when and where it’s visible.
Finally, seek high ground. You gain viewing time in an aircraft, a mountain top, or a tall building. You see this effect all the time, when tall objects remain sunlit even though it’s dusk at lower elevations. Use this to your advantage and see that selenelion!
References
- Beatty, Kelly (2010). “In Search of Selenelion“. Sky & Telescope.
- Espenak, Fred; Meeus, Jean. “Visual Appearance of Lunar Eclipses“. NASA.
- 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.