Dancing to Totality
A total eclipse of the sun would never be possible were it not for an extraordinary coincidence. Although the Sun’s diameter is 400 times larger than the Moon’s, the Sun is also 400 times farther away. This means that, on average, the Sun and Moon appear to be about the same size in the sky—but this alone doesn’t result in solar eclipses, let alone total solar eclipses.
As Earth orbits the Sun, the apparent diameter of the solar disk varies, because Earth’s orbit is slightly elliptical. When our planet is close to the Sun (January), the Sun appears about 3% larger than it does in July, when Earth is farthest from the Sun. The Moon’s orbit around Earth is also elliptical, only more so. When the Moon is closest to Earth (think “super Moon”), its apparent diameter is 14% larger than when it’s at its farthest. But this variability in the apparent size of the Sun and Moon still doesn’t give us solar eclipses.
The Moon and Earth dance around each other, while both swing together around the Sun. Once a month, the Moon passes between Earth and the Sun—a lunar phase called new Moon. This is the only time it’s possible to have a solar eclipse, but we don’t get a solar eclipse every new Moon. That’s because the Moon’s orbit around Earth is tilted by just over five degrees compared to the plane of Earth’s orbit around the Sun. So most times, when the new Moon crosses Earth’s orbital plane, it passes above or below the Sun, and Luna’s shadow misses Earth completely. But roughly every six months, the Moon crosses Earth’s orbital plane in front of the Sun. Now, finally, we see a solar eclipse. However, that eclipse is not always a total solar eclipse. In fact, over a span of 10,000 years, only 26.6% solar eclipses are total solar eclipses. Why is that?