Eclipses and World Building
I can go with your scifi/fantasy story’s super-impossible thing being associated with an eclipse. It’s activating or deactivating people’s super-powers? Sure! Certain magic spells can only be cast during an eclipse? Sure! The moon transforms into cheese? OK, whatever. (pun not intended)
But please, please get the basic mechanics right!
- The moon’s phase is caused by the current angle from the planet to the moon to the sun. Remember: the moon is a giant ball, and the side that’s facing the sun is the side that’s lit. The closer they are (visually), the narrower the crescent. The farther they are, the more of the moon is illuminated, until they’re opposite each other and the moon is full.
- The points of a crescent always point away from the sun.
- A solar eclipse can only happen during a new moon, because that’s the only time the moon can block the sun.
- A lunar eclipse can only happen during a full moon, because that’s the only time the Earth can block sunlight from reaching the moon.
- A lunar eclipse looks roughly (but not exactly) the same across the half of the planet that can see it, but keep time zones in mind: local daylight and distance from the horizon will vary.
- A solar eclipse is only visible from a small area, and that area moves during the eclipse, along with the moon’s movement and the earth’s surface rotating.
- If the moon gets farther away from Earth, its silhouette gets smaller and can’t cover the whole sun. This is how you get an annular (aka “ring of fire”) eclipse. Move it even farther out, and it’ll eventually get to the point where it just looks like a sunspot.
- If the moon gets closer, it will increase the area affected by the solar eclipse…but only up to a point. Imagine you’re trying to eclipse a streetlight using a golf ball as viewed on a grapefruit, and you’ll see there’s no way to make the shadow cover the entire surface of the grapefruit.
- In a realistic/science-fiction setting, moving the moon closer or increasing its mass also makes tides stronger. Too close and you end up with massive earthquakes and the the moon breaking apart!
- Even in a fantasy setting where you don’t worry about tides destabilizing the planet, there’s still a geometric limit to how close you can get that golf ball to the grapefruit.
- All of the above still apply if your fictional planet has more than one moon! If the second moon is 90 degrees away from the sun on the day of the solar eclipse, that second moon is going to be first or third quarter. If the second and third moons are slightly ahead and behind the one causing the eclipse, they’ll be crescents. And with a lunar eclipse, it’s only going to affect one of the moons unless two or more of them are full at the same time and close enough in the sky to fall within the planet’s shadow at the same time. (In the case of real-world Earth, its shadow at the moon’s distance is about 3 times as wide as the moon, so a fictional second moon would have to pass that close to it visually.)
