Hi, friends! As I’m sure many of you have heard, there is something incredibly spectacular happening this Monday, August 21st–a total solar eclipse! If you haven’t heard of it, or you aren’t sure exactly why this is significant, don’t worry! I’ve got you covered with today’s post on Everything You Need To Know About the Solar Eclipse! So let’s jump right into it, shall we!
What Is A Solar Eclipse?
A solar eclipse occurs when the Moon passes between the Sun and the Earth, blocking part/all of the Sun’s light as seen from Earth. This straight line configuration of 3 celestial bodies (the Earth, Moon, and Sun) is known as syzygy in astronomy. In a total eclipse, the disk of the Sun is fully blocked by the Moon, leaving only the fuzzy outer corona and any solar prominences visible around the edges.
In a partial or annular eclipse, only a part of the solar disk is covered by the Moon. The difference between the two being… A partial eclipse happens when the Sun and Moon are not exactly aligned with the Earth, and so only a portion of the Sun is covered by the Moon. While an annular eclipse happens when the Sun and Moon are exactly aligned with the Earth but the apparent size of the Moon is smaller than that of the Sun, producing a bright annulus ring (instead of full blockage) of the Sun around the Moon’s silhouette.
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How Often Does A Solar Eclipse Happen?
If the Moon were in a perfectly circular orbit, a little closer to the Earth, and in the same orbital plane, there would be a total solar eclipse every month. However, the Moon is in an elliptical orbit, whose distance from the Earth changes over the course of a month and is actually inclined 5 degrees to the ecliptic plane (the plane in which Earth orbits the Sun). So not only does the Moon’s apparent size at Earth change over time, but its shadow also misses Earth’s surface quite frequently (when the Moon is above or below the Earth ecliptic plane). This leads to the question…How often does a solar eclipse happen? And perhaps more fundamentally than that, What events must occur for a total solar eclipse?
Before diving into this question, it’s important to note that we live in a cosmically-convenient time when this type of event is possible. It just so happens that the Sun-Earth distance is approximately 400x that of the Earth-Moon distance and the Sun’s size is approximately 400x that of the Moon as seen by Earth. This means that both the Sun and the Moon appear to be roughly the same size in the sky from Earth (0.5 deg of arc) and so the Moon is able to just perfectly blot out the light from the Sun during a total solar eclipse. However, because the Earth-Moon system is anything but static, in a short 50 million years, the Moon will be too far away (too small in the sky from Earth) to produce such a total solar eclipse. And hundreds of millions of years prior to today, the Moon would have been much closer (and larger), and so an annular eclipse would not have been possible. So it’s really neat to think about the fact that our lives fell right into the sweet spot for both annular AND total eclipses to happen! Neat, no?! [ref]
Okay…so a total solar eclipse! Let’s talk about each of the events that must happen for one of these to occur!
Events That Must Occur for a Total Solar Eclipse:
1. New Moon:
First and foremost, the Moon must be positioned between the Earth and the Sun. This happens once a month during what’s called a New Moon. Because the Moon is positioned between the Earth and the Sun, there is no reflection emanating from the Moon’s surface as seen by Earth. Thus a New Moon is recognized each month with no visible Moon in the sky.
image, Lunar cycle showing the different phases of the Moon.
2. Ecliptic Plane Crossing:
Secondly, because the Moon’s orbital plane is tilted by 5 degrees with respect to the Earth’s orbital plane around the Sun, the Moon must cross this ecliptic plane during a New Moon. This crossing occurs across the line of nodes. It is considered an ascending node when the Moon crosses from South to North of the plane, and a descending node when the Moon crosses from North to South of the plane. A solar eclipse can occur during either type of crossing, but must occur when the Moon is aligned within ~15 degrees of the ecliptic plane. The tilt of the Moon’s orbit will be observable as the Moon moves across the sun from West to East in a diagonal.
image, Tilt of the Moon’s orbit must coincide with an ecliptic plane crossing during a New Moon for an eclipse to occur.
3. BIG Moon + SMALL Sun:
Both the Moon’s orbit about the Earth and the Earth’s orbit about the Sun are ellipses, and so the apparent sizes of both the Moon and Sun will vary over time. The magnitude of an eclipse is determined by taking the ratio of the apparent size of the Moon over the apparent size of the Sun. For a total eclipse, the magnitude is greater than 1 (BIG Moon, SMALL Sun); while for an annular eclipse, the magnitude is less than 1 (SMALL Moon, BIG Sun). To have a total eclipse, the Moon needs to be at its largest apparent size and the Sun needs to be at is smallest apparent size. This occurs when the Moon is closest to Earth in its orbit (perigee) and when Earth is farthest from the Sun in its orbit (aphelion). Earth is farthest from the Sun in July and closest to the Sun in January. This means that total eclipses are more likely to occur near July each year. If the Moon is not big enough to cover the full disk of the Sun but the 3 bodies are perfectly aligned, the result would be an annular eclipse. Annular eclipses occur for 60% of central eclipses.
Apparent sizes of the Moon and Sun for an annular and total eclipse.
4. Shadow Location on Earth:
And finally, the total solar eclipse is only visible along a narrow band (the width of the Moon’s shadow) as it tracks along the Earth. This width is determined by the apparent sizes of the Moon and the Sun. In general, the total eclipse shadow is only 100 miles wide, while the partial eclipse region can be much larger (4000 miles wide). However, because the Earth’s surface is covered by 71% of water, these events are likely to trace out a path over the open water, or a location that isn’t easily accessible to the masses. In this way, total solar eclipses are considered rare. You must be located within the umbra for a total eclipse or the penumbra for a partial eclipse. The United States hasn’t had an event this close to home in nearly 38 years! [ref]
image, Shadow geometry of the Moon onto the surface of the Earth during a solar eclipse.
Alright…so given that all of these things line up and a total solar eclipse occurs, how often does that happen? Well, the Moon crosses the ecliptic plane at two nodes (ascending and descending), which are 180 degrees apart. And a New Moon occurs close to the nodes at 2 different periods of the year, approximately 6 months apart (known as eclipse season). Thus, there will always be at least 1 solar eclipse during each of these periods. A minimum of 2 solar eclipses per year occur as a result. However, if the New Moon occurs close enough to a node during two consecutive months, then 2 separate partial eclipses will occur in both instances. Thus in a given year there could potentially be up to 5 solar eclipses. Note however that no more than 2 of these can result in a total solar eclipse and that 5 solar eclipses in a year is the maximum, not the norm. [ref]
Also note that while total solar eclipses occur at Earth every 18 months on average, it’s estimated that they occur at a given location only once every 360 to 410 years on average. So this is indeed a rare event for the United States in particular! If you have the chance to get out and see the solar eclipse, even if you are not in the path of totality, DO IT!
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When/Where Can I See It?
On Monday August 21st, the Moon’s shadow will track across the United States starting at 9:05am PDT time on the West Coast, traveling southwestern and ending at 4:09pm EDT on the East Coast. The dark inner shadow region known as the umbra is where the total eclipse will be visible. However, anyone in the lighter shadowed region known as the prenumbra (anywhere in North America, Central America, the Carribean, and northern South America) will still be able to see incredible views of the partial eclipse. You can check to see if you are inside the 70 mile-wide path of totality traveling across the United States and what times to expect the total/partial eclipse to happen in your specific area here or here. There’s also information for the eclipse by state here.
image, Map of Moon’s shadow path across the United States.
If you aren’t in the path of totality, don’t worry! There will be plenty of cool things to look at during the partial eclipse, as are detailed in the following section!
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What can I expect to see?
If you are viewing the entire solar eclipse event, you can expect the event to take roughly 2 hours, 45 minutes. Below is a brief schedule of events to follow. These are the phases you would expect to see if you are in the path of totality (within the umbra in the 2nd figure above). If you are not in the path of totality, you may still be able to see the partial eclipse. Much of the United States will have a fantastic viewing opportunity even if you are not in the path of totality. The following guide was developed from here and here.
Phases of a Total Eclipse:
1. First Contact: occurs when the edge of the Moon first touches the edge of the Sun.
image, Just after first contact during a solar eclipse, when the Moon first begins to block light from the Sun.
2. Partial Eclipse: occurs as the Moon moves over the disk of the Sun, resulting in a crescent Sun shape that can last over an hour.
Look along the edge of the Moon. Can you spot any jaggedness or roughness when comparing it to the edge of the Sun? If so, you are seeing the irregular, cratered surface of the Moon!
image, Partial solar eclipse.
As the Moon moves farther and farther over the Sun, notice the darkening of the sky and the cooling of the temperature around you.
The Sun will reduce to a tiny crescent sliver just before totality (~10 minutes prior). Look for wavy shadow bands against solid backgrounds (such as the side of a car or a piece of paper) in the final minutes before totality. Sunlight gets refracted through heat waves in the atmosphere, which can cause these waves. (The same thing that causes stars to twinkle in the night sky!)
Also look for brighter planets and stars to emerge as the sunlight continues to dim. Venus should be to the West of the eclipse, while Jupiter should be to the East.
image, Partial solar eclipse with the signature crescent Sun shape.
3. Diamond Ring: occurs when the Sun’s light converges to a bright ‘diamond’ point on one edge, while the Sun’s corona can be seen as a bright ring around the edge of the Moon.
image, Diamond ring effect as the last rays of light make it to Earth.
4. Baily’s Beads: occurs just before totality when the diamond point breaks up into little beads, as the Sun’s light shines through the low-lying valleys along the Moon’s surface.
image, Baily’s beads appear as the last light from the Sun makes its way to Earth through the valleys on the lunar surface.
5. Second Contact (Totality): the Moon covers the entire disk (photosphere) of the Sun and only the outer solar corona is visible along the edge.
Upon totality, the solar corona (outer atmosphere of the Sun) appears as a wispy halo of white streaks surrounding the Sun. Note that we are in solar minimum (of the 11-year solar cycle), and so the corona may appear relatively small and quiet.
Next look closer along the edge to see if you can see a pink-red layer, known as the chormosphere (literally ‘sphere of color’). Look for any solar prominences, observed as tiny red spots, arcs, or loops jutting out from the solar disk.
image, Total solar eclipse–the Moon is completely obscuring the solar disk.
6. Third Contact/Partial Eclipse: occurs as the Moon continues to move across the Sun, seen as a bright crescent beginning to form on the opposite side of the Moon from where the initial diamond ring was observed. (Another diamond ring may appear when this happens.)
Look again for a brief glimpse of the choromosphere and any solar promeninces during the last part of totality as it switches back to a partial eclipse.
image, Partial solar eclipse with the signature crescent Sun shape.
image, Partial solar eclipse.
7. Final Contact: occurs when the trailing edge of the Moon no longer overlaps the solar disk. This signals the end of the eclipse.
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How Can I Safely View the Eclipse?
Looking directly at any sliver of the Sun for even just a few seconds can cause permanent damage to the retina, most times without pain or awareness of damage for several hours after. Directly viewing the Sun through any kind of optical device, such as binoculars, telescopes, or cameras is extremely hazardous and can cause irreversible damage to your eyes (and sensors) within fractions of a second. Don’t risk it! (Learn from someone who learned the hard way here.) You can read more about using the proper eye protection here and here.
Only use properly designed and certified glasses with solar filters for direct viewing of the sun (available here). Use the guide given below to determine when it is safe to take off your solar glasses (only during totality). Note that it is NOT SAFE to use sunglasses to view the solar eclipse. However, there are other methods you can use to safely view the solar eclipse if you don’t have any solar glasses available. The safest method of viewing the solar eclipse is by using a pinhole camera, which is fairly easy to build. You can either use a pair of binoculars or even a colander to indirectly project the Sun onto a white piece of paper for viewing. Or you can make a pinhole projector out of a cereal box. This pinhole viewing method is safe for observing sunspots and eclipses.
image, How to safely view the solar eclipse using glasses with certified solar filters.
I will be safely viewing the solar eclipse this Monday from Boulder, Colorado, which is within 90% of totality. This means I won’t be able to see the full total eclipse, but should get a spectacular partial eclipse. You can also catch live streams of the solar eclipse on many different platforms here.