Today, I had the pleasure of witnessing the total solar eclipse, a twice-in-a-lifetime opportunity. Yes, I’ve already seen a solar eclipse, and I’ve seen a handful of lunar eclipses, but I had to see this eclipse too. If you’ve met me, you know I get excited when the sun aligns with some buildings, so of course I was going to drive 200 miles to see the sun align with the moon. It was spectacular, but plenty of people will share their life-changing experiences with the eclipse. Instead, here are a few things I learned about eclipses while observing this one.
You can safely observe a partial eclipse with just your hand and a flat surface. Curl your index finger so as to make a tiny hole, and let sunlight pass through it onto the flat surface. Congrats! You just projected an image of the sun, and with any luck, you’ll see a crescent-shaped sliver of sun instead of the normal circle. This hand projection uses the same principle as a pinhole camera. I was going to make a pinhole camera, but I forgot to bring the materials with me, so I opted for this instead.
Outdoor lens flares from the sun during an eclipse will be crescents instead of circles. Lens flares are just images of the light source caused by lens imperfections. Turns out, they don’t always have to be circles. Typically, the lens flare is the shape of the light source. Since the light source—the sun—will appear as a crescent, your lens flares will be crescents too.
Speaking of crescents, a solar crescent from an eclipse will never match the shape of a real crescent moon in the night sky. This is because of how the crescents are formed. For a crescent moon, we’re looking at a half-lit sphere, which is similar to looking at a moon-sized circle partially covered by a larger black circle. Constructing the shape requires two different-sized circles. For a solar crescent, we can use another two circle construction, except the black circle is the same size as the sun, not bigger, so the shape is different. There are more nuances to the full argument, which you can prove mathematically by comparing the distances between the corners of the crescents. Ultimately, solar crescents and crescent moons cannot look the same if the sun and moon are the same size in the sky.
During a total solar eclipse, you can see celestial bodies you would only see at night. For example, I saw Jupiter and Venus during totality today. Since I’m so used to thinking about when they’ll be above the horizon at night, I really wasn’t expecting to see them during the day. Even in the dark twilight of totality, with no time for my eyes to adjust, the two planets were very easily visible. I should have brought my telescope.
Typically in Celestial Navigation, I’ll discuss some ways in which we can use the stars to find directions and calculate locations, but so much of that relies on the parallel rays assumption. Light from space hits Earth in the same direction for everyone, no matter where you are. Today, however, that assumption breaks down. Total solar eclipses are focused on small areas. They take advantage of the fact that you are a single observer, looking out, so the rays are radial, not parallel, and you are at the focus. You aren’t meant to find anything during the eclipse. Instead, it’s a brief moment when the sun and the moon find you.