By a twist of fate, Earthlings get treated to front-row seats in one of the greatest shows in the solar system: Eclipses!
Look up. You may notice that the Sun and the full Moon appear to be about the same size in the sky. In reality, the Sun is much larger than the Moon (you could fit about 72 million Moons inside the Sun), but it's so much further away from the Earth, they look the same size.
If you've been fortunate enough to witness an eclipse, you know there's something unearthly about it. During a solar eclipse, there's a chill in the air and the animals are still. Day turns into night as the Moon passes in front of the Sun. A lunar eclipse is subtler, but lasts longer, as the Earth's shadow slowly tinges the face of the Moon red.
When eclipses occur, they're like super-charged new or full Moons. Solar eclipses can only occur during a new Moon, and lunar eclipses can only occur at a full Moon. (No cheating here.) But unlike a regular new or full Moon, during an eclipse the Sun, Moon and Earth must be in perfect alignment. Since the Moon's path across the sky is tilted about 5 degrees from the Sun's path, eclipses can only happen twice each year (sometimes with an extra eclipse thrown in as an encore).
All eclipses are not created equal. If a solar eclipse occurs when the Moon is at perigee (close to the earth in its orbit), we see a total solar eclipse. In a total eclipse, the solar disk is completely obscured by the Moon. But if the eclipse occurs when the Moon is at apogee (far away from the earth) then we see what's called an "annular eclipse". In this type of eclipse, the edges of the Sun remain visible, creating a brighter "ring of fire" around the Sun.
Eclipses happen in cycles. The two places where the Moon's and Sun's paths cross are called the Nodes of the Moon. The nodes slowly shift across the sky, taking about 18.6 years to move through all of the 12 signs of the zodiac. So about every 9 years (half of 18.6), you can expect to enter an accelerated time of change for your own sign.
Solar and lunar eclipses also create families. An eclipse family is called a Saros Series. About 70 to 80 geometrically similar eclipses occur almost precisely 18 years, 11 days and 8 hours apart, evolving over 1200 to 1400 years. The Saros Cycle number of an eclipse suggests something about the nature of the eclipse, based on the other eclipses in the same family.