Helping Aubie this week is Dr. Jean-marie Wersinger, a physics professor at AU.

Dear Taylor,

During a solar eclipse, the Moon passes between the Earth and the Sun. People at some parts of Earth will see the Sun totally disappear behind the Moon and the birds will stop singing believing that night is falling and some superstitious people will imagine all sorts of weird things.

The fact that the Moon may completely cover the disk of the Sun is rather intriguing as the Moon is much smaller than the Sun. The diameter of the Sun is 400 times the diameter of the Moon. So how can an object cover another one that is 400 times larger? Well, that is another interesting question. It turns out that the Moon is much closer to Earth than the Sun and that presently the “apparent” size of the Moon and the “apparent” size of the Sun are pretty much the same. This was not always the case and will not be true millions of years into the future. A very interesting “law of physics” is at work that makes the orbit of the Moon slowly drift away from Earth. The speed of this so-called “recession” is only 1.5 inches per year but it piles up to miles and miles over millions of years. The way this works is as follows. The Moon and the Sun produce the ocean tides on Earth. The friction of the water with the solid part of Earth slows Earth’s spinning motion down. As the Earth slows its spinning motion down (days and nights get longer,) and the Moon picks up a wider orbit around Earth. So a few million years from now, people on Earth will not see complete solar eclipses anymore as the distance between Earth and the Moon will have increased so that the “apparent” size of the Moon will be smaller.

Now to the second question. When the Moon passes between the Earth and the Sun, the face of the Moon we see from Earth will be completely dark. This is called a New Moon. Remember that the Moon shines only because the Sun lights it up and what we see is the part of the Moon that is lit by the Sun. Since the Moon passes between the Earth and the Sun the part of the Moon we see is dark.
except during a solar eclipse.

Thanks for your question!

Aubie and Dr. Wersinger