Many of you already know we are about to experience a couple of solar eclipses. Are you ready?
Protection for your eyes is a key element in preparing to view the Sun. I will get into that next time, but first a little about what’s going on with solar eclipses and why there are different types of solar eclipses.
Any eclipse is simply when the shadow of one body falls on another body. For our purposes it’s when either the Moon or Earth gets between the other body and the Sun. The Sun’s light is blocked, and a shadow is cast on the other body.
For a lunar eclipse, Earth’s shadow falls on the Moon, darkening it. For a solar eclipse, the Moon’s shadow falls on Earth. This article will focus on solar eclipses, and how to safely observe them.
The Moon’s orbit around Earth is nearly but not quite circular, it is an ellipse, so it gets closer to or farther away from Earth. The Moon’s orbit is also not exactly in the same plane as Earth’s orbit around the Sun (the ecliptic) so sometimes the Moon is above and sometimes below Earth’s orbital plane. Both factors play a role in solar eclipses and the type of solar eclipse we experience.
Another phenomenon is the shadow itself. There are two components of any shadow, the central dark component called the UMBRA, and the peripheral lighter component called the PENUMBRA. The above factors influence which part of the shadow we experience. Within the umbra we experience a TOTAL eclipse, within the penumbra we experience a PARTIAL eclipse.
Partial Solar Eclipse: The Moon is between the Sun and Earth, but we are in the penumbral shadow. With solar protective eyewear we can see the Moon’s dark disk in front of and partially blocking the Sun.
Total Solar Eclipse: The Moon is between the Sun and Earth, the right distance to fully cover the Sun, and we are in the umbral shadow. During the few minutes of totality we can observe without protection, and see the Sun’s corona.
Annular Solar Eclipse: The Moon is between the Sun and Earth but a little too far away and does not fully cover the Sun’s disk. The umbral shadow does not fall on Earth – it terminates some distance above Earth and results in what’s called an ANTUMBRAL shadow. With solar protective eyewear we see a dark circle with a brilliant ring surrounding. That’s why it’s called annular.
Hybrid Solar Eclipse: A very rare event. An eclipse morphs from annular to total or vice versa. The Moon’s distance is right at the transition point between total and annular. As the Moon moves in its orbit its distance changes enough to change the eclipse type along the eclipse path.
Next week – solar observing safety and a Sun Party on May 20.
What’s in the Sky?
May 6-9; an hour before sunrise; southwest: Watch a waning gibbous Moon move into Scorpius and Sagittarius.