Back To CourseBasics of Astronomy
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Back around 585 B.C., the Milesian astronomer, Thales, was taught by the Egyptians a lot of important stuff that was later used by Thales to predict a total solar eclipse that was about to occur. A solar eclipse is an event that occurs when the Sun's light is obscured as the Moon passes directly between the Earth and the Sun. This means the Moon is in the new Moon phase at this point.
And occur Thales' solar eclipse did, just as he apparently predicted. In fact, this eclipse occurred when the Persian and Lydian armies were fighting one another on a battlefield. They were so amazed by this eclipse, they decided to stop fighting and call it a day. Thus, the battle ended.
How these amazing, war-stopping eclipses occur will be peacefully settled for you in this lesson.
Be it the Persians, Lydians, or ourselves, we are pretty lucky we can even see a solar eclipse. It just so happens that the angular diameter of the Sun and Moon are both about half a degree. The angular diameter is the angle that's made by two lines starting at an observer and ending on the opposite sides of an object.
This basically says that the Moon has a size that's equal to that of the Sun as it appears on our sky. You must clearly realize that the reality is anything but. The Sun is actually around 400 times larger than the Moon but it's also about 400 times farther away. This makes it appear, on our sky, that the Sun and Moon are about the same size.
You can demonstrate this concept for yourself pretty easily. Take a big and a small plate from your kitchen cupboards. The big plate will be the Sun and the small one will be the Moon. Have one person hold the small plate close to you and another person walk slowly away from you with the big plate. There will come a point in space where the big plate will appear to be the same size as the small one.
What this means is that, in our sky, the Moon is just big enough to cover up the disk of the Sun and cause a solar eclipse.
If the Moon covers up the entire Sun, you get a total solar eclipse, but if only part of the Sun, you will see a partial eclipse. A total eclipse occurs gradually as it takes about an hour for the Moon to cover the Sun's disk and the Sun remains totally eclipsed (called totality) for only a few minutes, 2-3 minutes on average.
In the beginning of what will become a total eclipse, the Sun will look like it's being eaten from west to east. During the partial portion of the eclipse, the sky will darken. It will seem like nighttime is about to start.
The temperature will drop and birds will head to their nests for the approaching 'night.' As the amount of sunlight grows smaller as more and more of the Sun's disk disappears from view, you'll see the brightest stars appear in the sky.
As the Moon's umbra, the portion of the Moon's shadow that is completely shaded, passes over your location on Earth, you will see a total eclipse! Thus, you are in the Moon's total shadow and cannot see the Sun's surface.
If you were located in the Moon's penumbra, the portion of the Moon's shadow that is partially shaded, you would see some of the Sun sticking outside the edge of the Moon and thus you'd only experience a partial solar eclipse. If you were standing outside the Moon's penumbra on Earth, then you'd never see any eclipse whatsoever, partial or otherwise.
You can sort of recreate the umbra and penumbra by taking your index finger and sticking it under a desk lamp that's shining directly downwards onto the desk. Put the finger somewhere in between the desk and lamp as you do this. You see how there's a portion of the shadow of your finger that's really dark, right at the center? That's like the umbra. Then you see how just outside of that is a partially shaded area? That's like the penumbra.
When the Moon covers ups the visible surface of the Sun, the photosphere, during a total eclipse, it reveals the following:
And, while all of these things are pretty cool and pretty cool-looking, make sure to never ever stare at a solar eclipse without some very heavy-duty eye protection specifically designed for this event or else you can seriously damage your vision forever.
A solar eclipse is an event that occurs when the Sun's light is obscured as the Moon passes directly between the Earth and the Sun. If you were located in the Moon's penumbra, the portion of the Moon's shadow that is partially shaded, you would experience a partial eclipse. If the Moon's umbra, the portion of the Moon's shadow that is completely shaded, passes over your location on Earth, you will see a total eclipse!
We are lucky to even experience a solar eclipse to begin with. It can happen because the angular diameter, the angle that's made by two lines starting at an observer and ending on the opposite sides of an object, is basically the same for the Moon as it is for the Sun.
When the Moon covers up the visible surface of the Sun, the photosphere, during a total eclipse, it reveals the following:
Once the video is over, you should try to:
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons