Eclipse

2024 Eclipse resources from our friends at Rice University

All About Eclipses

A solar eclipse occurs when, from our perspective on Earth, the Moon moves between us and the Sun and blocks our view of it. From a space-based perspective this happens when the Moon’s shadow lands on the surface of the Earth. Observers in the shadow experience an eclipse.

As the Moon orbits around Earth, we see different parts of its day side, this creates the phases of the Moon. The new moon phase is the time when the Moon is between the Earth and the Sun, and the day side is facing entirely away from us. This happens every month. However, solar eclipses do not happen every month. The Moon’s orbit around the Earth is inclined relative to Earth’s orbit around the Sun. Most months the new Moon passes above or below the Sun and there is no eclipse. About twice per year the orbits line up and an eclipse is possible. But the Moon has to be near the new moon position when that happens or there will not be a solar eclipse.

The Sun is not far enough away to look like a point of light, like the stars do, it has a size in the sky. Because of this the shadow cast by the Moon has two zones:

An umbra where all of its light is blocked
A penumbra where only some of its light is blocked

If you observe the Sun and Moon from within the penumbra you see a partial eclipse of the Sun. If you observe the Sun and Moon from within the umbra you see a total eclipse of the Sun.

Partial Eclipse

During a partial eclipse of the Sun, a piece of the Sun is blocked by the Moon. It may not get noticeably dark unless a large fraction of the Sun (more than 80%) is blocked. The change in brightness may feel like when a cloud passes in front of the Sun on a partly cloudy day.

Whenever any part of the Sun’s photosphere is visible it is not safe to look at it with your eyes. The ultraviolet light from the Sun can permanently damage your eyes. You can use welder’s glass (#14, available at most hardware stores) to safely view the Sun. Welder’s goggles or a welder’s mask with with #14 glass installed in the viewer is an effective way to safely view the Sun. Special eclipse glasses are sold to view the Sun as well. Look for these from reputable vendors only. You should not be able to see anything but the Sun through a set of eclipse glasses.

Small telescopes also concentrate enough visible light from the Sun to start a fire at the focus. Do not look through a telescope or binoculars pointed at the Sun without proper filters. Use filters specifically sold for solar viewing in small telescopes.

When the Sun is almost completely hidden by the Moon, the visible light brightness becomes dim enough that people will be tempted to look at it. But the UV brightness is still dangerous at this point. Never use regular sunglasses to look directly at the Sun. They do not provide adequate protection.

A wonderful way to view the Sun during a partial eclipse is to project its image. The simplest way to do this is with a pinhole projector. This can be as simple as a piece of cardboard with a tiny hole in it. It can even be your hands with the fingers criss-crossing to let tiny bits of light through. The image projected through the tiny holes is in fact the Sun. During the partial eclipse, you will see the shape of the projected image become a crescent shape as some of the Sun’s disk is blocked by the Moon.

You can also project larger and brighter images of the Sun with small telescopes. The image can be projected onto a large white poster board or similar. This is very nice as it allows many people at once to see the Sun, and you can often make out features on the surface of the Sun, like sunspots. Just take care to always monitor the telescope to be sure nobody looks through it.

During a partial eclipse it is fun to take note of shadows all around you. They will get very fuzzy the more crescent shaped the Sun becomes.

Annular Eclipse

There is a special kind of partial eclipse, called an annular eclipse. During this type of eclipse the Moon does not completely cover the face of the Sun, but it goes entirely inside the disk of the Sun and leaves a ring of light (an annulus; which is Latin for ring) during the middle of the eclipse. The effect is sometimes nicknamed the “ring of fire.” It is a very pretty effect and a lovely spectacle of nature to witness.

This happens because the Moon orbits around the Earth in an ellipse, so it is sometimes closer to us than average and sometimes farther than average. When an eclipse happens when the Moon is farther away than average, it will not be big enough to cover the whole Sun. The umbra part of its shadow will not reach Earth’s surface.

Total Solar Eclipse

We live in a special time in the history of the Earth. The Moon is about 390 times smaller than the Sun, and right now in Earth’s history it is about 390 times closer to us than the Sun. In the past it was closer and in the future it will be farther away. So right now the Moon appears to be the same size in the sky as the Sun. That means that the Moon can exactly cover the Sun’s disk in a special eclipse called a total solar eclipse.

When the Moon exactly covers the Sun, the sky becomes dark like night. A twilight glow appears all around the horizon. The temperature drops. Some bright stars and planets will become visible in the sky. But most spectacularly of all, the atmosphere of the Sun becomes visible. The chromosphere is visible as a red circle around the eclipsed Sun. In binoculars you may even spot coronal loops and prominences. The corona of the Sun appears as bright wisps and streamers emanating from the Sun and stretching far from it. The name corona is Latin and means “crown.” It got this name because of its appearance during total eclipses of the Sun. It looks like a spectacular crown of light spread across the sky. It is a truly awesome sight, something that no picture can adequately capture. Human eyesight is both wide-field and high dynamic range, and able to take in the bright parts of the corona near the surface of the Sun and the dimmer parts stretching far away from it. During the total eclipse it is safe to look at the Sun without any filters or special glasses. In fact, a simple pair of binoculars are great for looking at the chromosphere and inner corona. Total eclipses usually last for a few minutes. Once it ends, we are back to a partial eclipse and need to practice safe solar viewing.

Upcoming Eclipses

In some ways, solar eclipses are not very rare. They happen almost twice per year somewhere on Earth. But the Moon’s shadow is relatively small compared to Earth, so not everywhere on Earth gets to see an eclipse when it happens. The penumbra is larger and more of Earth may see a partial eclipse, but to see an annular or total solar eclipse you need to be right in the small path of the Moon’s shadow. For any particular spot on Earth it can be an average of over 300 years between total solar eclipses. So it is often necessary to travel to see an eclipse within your lifetime.

The United States has been lucky recently. We had a total solar eclipse pass through the country from coast to coast in 2017. On October 14, 2023 an annular solar eclipse will cross much of the country from Oregon to Texas. And on April 8th, 2024 a total solar eclipse will cross the country from Mexico into Texas through New England into Canada.

NASA’s eclipse website gives details about these upcoming eclipses and how you can travel to see them or watch livestreams from observers who will be in the path of both eclipses.