The great American solar eclipse is just over a month away, and we are super excited. We have interactive maps and location charts to help you find the best place for viewing the totality. However, a pressing question remains – how long will the totality last on August 21, 2017?
We wish we could give you a straight up answer, but the truth is, there isn’t any. This is the first time in 38 years that the continental US will witness a total solar eclipse. The path of totality is a whopping 70 miles long and the time duration of totality will vary greatly depending on where you are during the eclipse.
If you are standing right on the edge of the path, you will see only a couple of seconds of totality. If you can position yourself in the very center of the path, you may even see 2 minutes and 42 seconds of totality.
How to get maximum totality?
For understanding how you can maximize your chances of observing a prolonged totality, you need to know how an eclipse works. We see a solar eclipse when the moon comes in between the sun and the earth. There are two types of shadows cast by the moon on the surface of the earth – Umbra, and penumbra.
The path of totality is the path the umbra will take during the solar eclipse. If you position yourself outside the path, you will be in the partial shadow of the penumbra. In that case, you will not experience complete darkness.
The umbra projects out into the space between the moon and the earth like a tapering cone. When the moon is closest to the surface of the Earth, the umbra measures about 170 miles across. The shadow will average to only about 68 miles or 109 km wide on the day of the great American solar eclipse August 21, 2017.
Why is the duration of totality varying?
Another thing you must not forget – the shadow will be mobile. The moon’s shadow will move as fast as the moon. The speed of the umbra will average to about 2,300 mph or 3,700 km/h on the surface of the earth. At this time, we must also take into account two more factors –
- The earth is a sphere with a surface curvature
- The earth rotates from west to east direction
Since the earth rotates in the same direction as the moon, any particular viewing point on Earth will “race” the moon towards the east. Therefore, the shadow during totality will move as fast as 2,288 mph or 3,683 km/h at the equator. Since the earth’s surface is not flat, the moon’s shadow will move the slowest along the center of the path of totality.
Slowest speed, longest view
Tennessee will witness the slowest speed of the umbra as it approaches the mid-point of the path right here. The averaging speed will be 1,323 mph or 2129 km/h. The longest duration of totality will be around 2 minutes and 40 seconds. In total, the moon’s shadow will graze along the continental US from Oregon to South Carolina during the great American eclipse in about 90.7 minutes.