Your Definitive Guide to Solar Eclipses

solar eclipses
Three types of solar eclipses: (left) a total solar eclipse image taken in 1999 by Luc Viatour (used by permission); upper right is a partial eclipse, taken by Tom Ruen in 2014 (CC BY-SA 2.0); and lower right is an annular eclipse taken in 2012 by SMR. Geog, available by Wikimedia Commons, Share-like 3.0. Composite made by Carolyn Collins Petersen

Solar eclipses are natural events that occur on several worlds in our solar system when a moon's orbit takes it between its planet and the Sun and blocks the Sun for a short time. The moon casts a shadow that travels in a path across the planet's surface, and anyone inside that shadow would see a partially or fully blocked Sun.

Of course, the eclipses we are most familiar with are those we see from Earth. They happen as our own Moon orbits the planet (which is itself orbiting the Sun). Occasionally, its path puts it directly in line with the Sun, and that sends a shadow sweeping across some part of Earth's surface. Interestingly, the Moon experiences a solar eclipse during a lunar eclipse. That's because Earth is passing between the Moon and the Sun, and Earth's shadow darkens the Moon. 

Solar eclipses on Earth occur in cycles, and only during the lunar phase called the "new moon". An eclipse doesn't happen every time, due to the tilt of the Moon's orbital plane when compared to Earth. However, when everything lines up, then we get a solar eclipse that darkens a small swath of the planet called the "path of totality".

Viewing Solar Eclipses from Earth

Because solar eclipses are easily trackable and predicted for well into the future, people can make plans to travel to view them, particularly for total eclipses. They're amazing to watch and are well worth the effort. Let's look at the timeline for a total solar eclipse as an example of eclipse-gazing. If you're planning to see a total solar eclipse for yourself, the next ones are July 2, 2019 (visible from extreme southern north America and much of South America), June 21, 2020 (visible from parts of Europe, Asia, Australia, Africa, and Pacific and Indian Oceans), December 14, 2020 (South Africa, South America, and other southern locations). The next total solar eclipse visible in the U.S. is April 8, 2024. 

First Contact

Each total solar eclipses goes through four steps. When the Moon first begins to block the sun, that's called "first contact". It can last up to an hour or so. As the Moon covers more of the Sun, the atmosphere in the path of totality (the deepest shadow) starts to darken noticeably. People outside totality may see some lesser amount of twilight.

The air temperature starts to cool down. During this time, it's not safe to view the Sun directly, so observers need to use good eclipse goggles or solar filters on their telescopes or binoculars. NEVER look directly at the Sun during this time and DO NOT look at it through a telescope without a filter. Doing otherwise will harm your eyes and cause blindness. Really, it's never a good idea to look directly at the Sun, eclipse or not. 

Second Contact

When the Moon starts to completely block the Sun, that's called "second contact", or "totality". Right as totality begins, people look for a bright flash as the last of the Sun's light flashes around the Moon and through its mountains. It looks very much like a diamond and the eclipsed Sun looks like a ring. For that reason, eclipse-chasers call this the "diamond ring" effect.

Totality is the ONLY time that it's safe to take off your eclipse shades to look at the Sun. It will be very dark outside, and the only thing you'll see is the blocked Sun, surrounded by its outer atmosphere. You might also be able to spot a few bright stars and planets in the darkened sky. The period of totality lasts for just a few minutes, so take in all the sights and sounds while you can.

Third Contact

At the end of totality, the Moon  "unblocks" the Sun. At that point, viewers need to put their eclipse glasses back on and keep an eye out for a possible second "diamond ring". The sky will slowly get brighter as the eclipse progresses, and temperatures will rise again. This part lasts for about another hour.

Fourth Contact

Finally, the Moon completely unblocks the Sun and continues on its merry way. This is called "fourth contact" and it's the end of the eclipse. Time to party! (Or, if you took pictures, time to process and upload them!)

Safety Advice

As mentioned above, viewing an eclipse can be done safely using eclipse goggles and/or filters on your telescope or binoculars. Good filters will let you see the Sun, and nothing else. If you hold them up to a light bulb and see the bulb, they're not good enough for solar eclipse viewing. These same goggles are extremely useful during partial and annular eclipses (when the Sun is not completely covered). You can also view an eclipse using the projection method.

The Mechanics of a Solar Eclipse

How does an eclipse happen? There are several things happening that contribute to one of these awe-inspiring events. The first is the Moon's elliptical orbit around Earth. The second is Earth’s elliptical orbit around the Sun. They provide a sort of clockwork motion that brings the three objects in line with each other.

In addition, the Sun and Moon appear to be the same size in the sky as seen from Earth, even though the Moon is very close to us and the Sun is 1.5 million kilometers away. The Sun is much larger than the Moon, but its distance makes it appear smaller than the much closer (but smaller) Moon.

Each month, the Moon's changing position with respect to the Sun causes its shape to appear to change . Astronomers call these changes the phases of the Moon. New Moon is the first phase each month. During New Moon, if the Moon and Sun align correctly and the Moon’s shadow hits Earth’s surface, some portion of the Sun will be blocked from view. This is a solar eclipse.

 A solar eclipse can only take place when the New Moon occurs close to where the Moon’s orbit crosses the ecliptic (the plane of Earth’s orbit around the Sun). This usually happens at least twice a year. In some years, up to five solar eclipses have occurred. Not every New Moon results in an eclipse. Sometimes the eclipse shadow misses Earth altogether.

Solar Eclipse Types

There are four types of solar eclipses, each determined by how much of the Sun is obscured by the Moon. The first and most spectacular is the total eclipse. That's when the Sun is completely obscured from view for a short period of time usually just a few minutes). The Sun’s intense light is replaced by a dark silhouette of the Moon. The corona (the superheated outermost solar atmosphere) stretches out around the eclipsed Sun, giving the scene a ghostly appearance.

The Annular Eclipse

The elliptical orbit of the Moon around our planet plays a role in whether a solar eclipse will is a total one. This is because the Moon can only appear larger than the Sun and cover it when it is closer to the Earth (near its perigee). If it's not, then an annular eclipse occurs. Like total solar eclipses, annulars happen when the Sun and Moon are exactly in line, but the Moon appears smaller because it's slightly farther away from Earth. 

The Partial Eclipse

The third and most common type of solar eclipse is the partial eclipse. It occurs when the Sun and Moon are not completely aligned and the Sun is only partially obscured. Unlike a total or an annular eclipse, these are visible over large portions of the Earth because they are caused by the moon’s penumbral shadow.That's the faint outer shadow that extends out from the umbral shadow you see during a total solar eclipse. Partials are common not only because they are viewable from numerous places on the globe, but also because they can occur even when the umbral shadow never reaches Earth’s surface.

The Hybrid Eclipse

The final type of solar eclipse is the hybrid eclipse. This is a combination of a total and annular eclipse that takes place when a total eclipse changes to an annular eclipse or vice versa along different sections of the eclipse’s path.

Solar Eclipse Frequency and Predictions

Each year, Earth experiences an average of 2.4 solar eclipses. The actual number can range from two to five, although, it is rare to have five. The last time five solar eclipses occurred was in 1935 and the next will not be until 2206. Total eclipses are the rarest and there is only one that happens every one to two years. Predicting them allows scientists and eclipse chasers to plan observing expeditions around the globe far in advance.

Edited and updated by Carolyn Collins Petersen.