The Upcoming October 14, 2023, Annular Solar Eclipse

The Twilight Zone: Terminating Umbrae and Penumbrae

The earth, moon, and sun are spherical. At the distance the earth and moon are from the sun, the sun’s light illuminates one-half of both the earth and moon. The division between the lit and unlit portions of either the earth or moon is called the terminator. The moon’s terminator is very stark, as can be seen by looking at the moon at any lunar phase other than full. On the other hand, the atmosphere of the earth scatters sunlight into a bit of the dark half of the earth so that the earth’s terminator is a bit fuzzy. This twilight zone where the sun is either rising or setting encircles the earth. The circle of a body’s terminator is in a plane perpendicular to the direction of the sun. Since the sun is much larger than either the earth or moon, the shadows of the earth and moon taper away from the sun. Thus, their shadows form cones, with the base of the cones being the terminator and the apex opposite the direction to the sun. We call either of these shadows the umbra (the plural is umbrae).

Since the sun’s angular diameter is about ½ degree, the angles at the apex of the umbrae of the earth and moon are about ½ degree. The distance to the sun varies by about 3% throughout the year, so the sun’s angular diameter and that angle at the apexes of the umbrae varies by 3% too. The moon orbits the earth each month, so throughout the month, the angle at the apex of the moon’s umbral cone varies an additional small amount. One-half degree is a very small angle, so the umbrae of the earth and moon are very long cones. The earth’s radius is about 8,000 miles, but its umbra extends more than 900,000 miles. The moon is only about a quarter of the earth’s size, so its umbra is only about 240,000 miles long. This is only slightly greater than the average earth-moon distance, so the moon’s umbra just barely reaches the earth’s surface. That is why the path of totality in the April eclipse will be about 115 miles wide. That isn’t very wide, but it is significantly wider than the path of totality during the 2017 solar eclipse (70 miles). Why the difference? The moon’s orbit is an ellipse, not a circle. Consequently, the distance between the earth and moon changes by about 14% throughout the month. The moon was slightly farther away from the earth during the 2017 eclipse than it will be during the 2024 eclipse.

The moon also has a partial shadow called a penumbra (the plural is penumbrae). The moon’s penumbra is a truncated cone, with the orientation reversed from the umbral cone—the apex of the penumbral cone points toward the sun, while the apex of the umbral cone points away from the sun. The apex of the penumbral cone is truncated by the circle of the moon’s terminator. The base of the penumbra extends indefinitely away from the sun. During a total solar eclipse, the umbra and penumbra both pass over a portion of the daylit half of the earth. But only in the moon’s relatively small umbra is a total solar eclipse seen. At locations within the much larger penumbra, one sees the sun only partially blocked out. Even on the path of totality, totality is preceded and followed by a partial phase.

In a total solar eclipse, the moon’s umbra is a cone with its base being the moon’s circumference and its apex (tip) barely reaching the earth. As the moon moves in its orbit around the earth, the moon’s umbra races across the earth’s surface. Locations that the umbra passes over experience a total solar eclipse for at most a few minutes. The much larger penumbra extends on either side of the umbra. Locations in the penumbra experience a partial solar eclipse. If the moon is near apogee during the eclipse, the apex of the moon’s umbra falls short of reaching the earth. Locations in which the extension of the moon’s umbra passes over will experience an annular eclipse, with the slightly larger appearing sun shining around the silhouette of the slightly smaller appearing moon, producing a ring, or annulus, of light around the moon, as shown at the top of the page. Note that the sizes and distances between the sun, moon, and earth are not to scale. Image credit: Sanu N, CC BY-SA 4.0, via Wikimedia Commons.

What Is an Annular Eclipse?

While the amount of sunlight one receives under an annular eclipse is greatly reduced from normal, the sky does not get dark, nor do brighter stars appear.

If the moon is near apogee, the point on its orbit farthest from the earth during a solar eclipse, the moon’s umbra fails to reach the earth. When this happens, the moon appears slightly smaller than the sun, and it fails to completely cover the sun, leaving a bright ring of sunlight around the moon’s dark disk. We call such an eclipse an annular eclipse, from the word annulus, meaning ring. There are marked differences between annular eclipses and total eclipses of the sun. For one thing, unlike totality, it is not safe to look at an annular eclipse without proper filtering. While the amount of sunlight one receives under an annular eclipse is greatly reduced from normal, the sky does not get dark, nor do brighter stars appear. Still, annular eclipses are worth seeing, if the effort required is not too great.

I’ve seen one annular eclipse, on May 30, 1984. Greenville, South Carolina, USA, was on the path of annularity. Since we had many friends there, my wife and I traveled to Greenville to see the eclipse. The path of annularity was very narrow, indicating that this eclipse was very close to being total. We were in the centerline of the eclipse and experienced only 11 seconds of annularity. Since the sun was 99.9% covered, streetlights came on. We saw shadow bands for several minutes, something normally seen at total eclipses. We saw Venus for 20 minutes. It was quite an experience. A friend used the same camera I had used five years earlier at the 1979 total solar eclipse to photograph the annular eclipse. Unfortunately, the exposure time was too short, so the images were very faint.

My Plan for This Annular Eclipse

Since I don’t have any good photographs of an annular eclipse, I’ve decided to go to Albuquerque to watch the October 14 annular eclipse. I’ll be doing this at Providence Bible Presbyterian Church. Besides the Saturday morning eclipse, I’ll speak at the church Wednesday evening before the eclipse and again Sunday morning after the eclipse. If you live close to Albuquerque, why don’t you join us?

As a side note, Canyon Ministries, with whom Answers in Genesis joins in our Grand Canyon raft trips, has a Grand Canyon/solar eclipse tour planned for October 14. While the eclipse will not be annular there, it will be a deep partial eclipse. It will offer a unique opportunity to combine the grandeur of Grand Canyon, Wupatki Indian ruins, and a partial solar eclipse. You can sign up for that here.