The New Moon
A solar eclipse can only occur during the lunar phase known as the New Moon. When the moon is new, it rises and sets with the sun because it lies very close to the sun in the sky. When it passes between Earth and the sun, the moon’s shadow falls on Earth’s surface creating a partial or total eclipse.
One in a Billion ... Trillion
The sun is just one of over 100 billion stars (or more) in our galaxy, the Milky Way. It used to be thought that the Milky Way was just one of 200 billion galaxies in the universe, but in 2016 NASA's Hubble Space Telescope discovered that as many as 10 times that number of galaxies actually exist. Space is crowded.
Going the Distance
The sun is the biggest star at the center of our solar system – approximately 400 times larger than the moon. How then does the moon manage to cover all of it during a solar eclipse? Coincidentally – or perhaps not – the moon is 400 times closer to Earth than the sun. This means that from Earth, the moon and the sun appear to be roughly the same size in the sky.
Living in the Past
Light from distant stars and galaxies takes so long to reach us that we are actually seeing these objects as they appeared in the past. As we look up at the sky, we are really looking back in time. For example, the sun's light takes an average of 8 minutes and 20 seconds to travel to Earth, so we see the sun as it looked 8 minutes and 20 seconds ago.
A Rare Occurrence
There are only two-five solar eclipses per year and total solar eclipses are even more rare – happening only once every 18 months. That may still sound like a lot, but most total solar eclipses are only visible from less than half a percent of the earth's surface. This is due to the moon’s varying orbital path around the earth.
The moon orbits the earth once every 27.322 days. It also takes approximately 27 days for the moon to rotate once on its axis. As a result, the moon does not seem to be spinning but appears to observers from Earth to be keeping almost perfectly still.
Catch It Quick
A total solar eclipse typically only lasts for a few minutes but the longest solar eclipse (to be viewed on Earth) has yet to happen. Scientists predict that in the year 2186, a total solar eclipse sweeping across Colombia, Venezuela, and Guyana will last approximately 7 minutes and 29 seconds.
The moon’s orbit grows about 1.5 inches larger every year, taking it farther and farther away from Earth. Once the moon’s growing orbit takes it approximately 14,600 miles (23,496 km) away from Earth, it will always be too far away to completely cover the sun, but, at the rate of current growth, that should take another 600 million years.