How do scientists predict celestial events? I mean, if the solar eclipse is happening today, how do they know it happened XX years ago or that it will happen XX years later?

I am not talking about simply predicting a solar eclipse but rather specifically how long it's been since this exact same type occured.

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nmxt t1_itpy6if wrote on October 25, 2022 at 1:32 PM

All celestial events occur in cycles, so whenever a cycle is complete, the same events occur again. In case of solar eclipses, the cycle is about 18 years (called a “saros”). This is a period of time in which the Moon and the Sun both complete whole numbers of their own cycles, setting them to repeat the same events all over again.

Comprehensive-Ad3963 t1_itq1iju wrote on October 25, 2022 at 1:57 PM

The moon completes an orbit every 27 days.
The Earth completes an orbit around the Sun every 365.2422 days (it may help to pretend the Sun orbits the Earth)

A solar eclipse happens when a line from the Earth to the Sun intersects with the Moon.

We can figure out when that happens using some circle geometry (we can get better predictions using ellipses):

Pull out a piece of paper and draw two circles, one inside the other and each having the same center. The outer one represents the Sun's fictional orbit around the Earth, and the inner one represents the Moon's actual orbit around the Earth. Draw 3 dots, one at the center of the circles, such that the three dots form a line. The outermost dot is the Sun, the next outermost the Moon, and the innermost the Earth. This is a total solar eclipse.

The Sun "moves" 2*pi radians over the course of 365.2422 days, while the Earth moves that many radians over the course of 27 days.

candidateforhumanity t1_itq27wb wrote on October 25, 2022 at 2:02 PM

You may have observed that the sun rises every 24 hours. You think it's safe to predict that it will rise again tomorrow. Maybe you talk to people that have noticed it before you and they confirm your suspicion: the sun has been rising everyday in the past too.

The sun rising is a cyclical celestial event like any other.

Lithuim t1_itpy94q wrote on October 25, 2022 at 1:33 PM

First you start with observations. People watched the skies for centuries, out of both boredom and religious intent. They meticulously recorded events.

After a while it became apparent that major events weren’t simply random - solar and lunar eclipses seemed to come in clumps, and always correlated with full or new moons.

With enough data on this, medieval astronomers could start to work out the mathematics behind it - the Earth and the Moon have very predictable orbits and will repeat the same cycles over and over again.

Today the orbits are well modeled, and we can predict eclipses with extremely high accuracy.

Loki-L t1_itq4bvt wrote on October 25, 2022 at 2:17 PM

Modern scientist can do simulations and stuff, but long ago people could figure that out relatively easily.

People write down stuff like seeing a solar eclipse it is pretty memorable.

If they write down when they saw an eclipse it gets relatively easy to see a repeating pattern.

If you pay a lot of attention you notice things like all solar eclipses happening during a new moon and all lunar eclipses happening during a full moon.

Next you notice that sometimes the moon during a new moon is above the sun and sometimes below it and that it only gives you an eclipse when it is directly in front of it. You can watch the moon move up and down in relation to the sun and you figure out that there are two repeating cycles involved, one of the phases of the moon and one of the moon going up and down. When both the moon going through a node and the moon going through a full or new moon coincide you get an eclipse.

It doesn't take much to predict when there will be a new moon and a full moon many years in advance and figuring out the other bit also just requires a bit of paying attention.

Then the rest is just a bit of math.

There is an 18 year cycle (223 times the moon through its phases) after which the pattern for possible eclipses repeats. So once you know about enough eclipses happening in the past you can just extend the known pattern forward even without understanding the physics or geometry or math behind it.

The Eclipse today matches the one that happened on October 14 in 2004 and the one that will happen November 4, 2040.

You can just always go backwards or forwards 18 years and 11 days (plus or minus a day depending on how many leap years there were).

Upstairs-Ad898 t1_itqufp9 wrote on October 25, 2022 at 5:09 PM

cool explanation

MuggleWitch OP t1_itrdwcd wrote on October 25, 2022 at 7:13 PM

Thanks, this is very detailed and pretty cool! I also wanted to understand how this would work in case of 'once is 800 <placeholder for absurdly long years> type occurence. Ex: Jupiter being closest to the earth only now. But I am guessing the same math logic would apply to that