How the moon’s shadow creates varying types of solar eclipses
By Hannah Halusker
The waxing and waning of the moon appear to us in phases over the course of a lunar month, which takes approximately 29 days to complete. One half of the moon’s surface that is facing towards the sun is always illuminated, but as the moon’s orbit brings it around the Earth, we see different portions of that illuminated half, creating the phases of the moon. When the side of the moon that faces Earth is unilluminated, we see a New Moon. When the portion of the moon’s surface we see is partially illuminated, we see a Crescent, Quarter, or Gibbous Moon. Finally, we see a Full Moon when the entire illuminated half of the moon is visible from Earth.
A solar eclipse occurs when the moon passes between the sun and the Earth, casting a shadow onto our planet. Solar eclipses can only happen at the phase of a new moon.
There are four types of solar eclipses, all dependent on which part of the moon’s shadow is being cast: the umbra or the penumbra. The umbra is the darkest part of a shadow. The penumbra is the partially shaded outer regions of a shadow.
This image depicts the moon’s cone-shaped, inner umbra, which is extended by the antumbra. The faint penumbra surrounds the umbra.
Image Credit: Umbra01 by Daniel M. Short / CC BY-SA 2.5
Here are the four different types of solar eclipses:
- A total solar eclipse – the type that will occur on August 21 in Clemson – happens when the point of the moon’s cone-shaped umbra is cast upon Earth and slides along the surface in conjunction with the moon’s movement. In every place located along this narrow path, a total solar eclipse will occur, lasting about three hours. Halfway through the eclipse, the moon will totally cover the sun for up to eight minutes. This is called totality. During this brief time, the corona – an aura of plasma that surrounds the sun – will become visible. The darkness of totality mimics deep dusk. Thus, grasshoppers might start to chirp and flower blooms might close. The temperature will drop by a few degrees, as well, though how much cooler it gets varies with each eclipse.
- A partial solar eclipse is the result of the moon’s penumbra falling over the Earth. In fact, the word “penumbra” is Latin for “almost shadow.” During a partial eclipse, viewers will only see a chunk of the moon blocking the sun. However, the closer you are located to the path of the penumbra, the greater the obstruction.
- When the moon is too far away from Earth to obstruct the entirety of the sun, an annular solar eclipse occurs. An annular eclipse resembles a total eclipse, in that the moon passes directly over the center of the sun. However, the difference is that the moon’s umbral cone cannot reach Earth during an annular eclipse. Instead, viewers are standing in what is called the moon’s antumbra, an extension of the umbra. Thus, the sun appears as an annulus, or a lopsided ring of light, from behind the moon.
- Depending on where you’re located, a hybrid solar eclipse can look like an annular or total eclipse. This is because the tip of the moon’s umbra is just shy of reaching Earth’s surface. A hybrid eclipse begins as annular, and then as the Earth rotates, it intercepts the umbra, allowing it to fully touch Earth’s surface in some areas before appearing again as an annular eclipse near the end of its path. Only about 5 percent of reported solar eclipses have been hybrid, making them the rarest of all four types.