Thursday, April 15, 2021

How far can you see?

Looking East, 9:00PM, October 14.

Looking East, 9:00PM, October 14.

By Mike P. Usher 

Tonight Jupiter is finally visible at a reasonable time, but still too close to the horizon for good binocular viewing. Also the Moon is still nearly full tonight – which washes out all the fainter stars, but it’s a good time to try a simple experiment. Everyone is familiar with a huge Moon floating just above the horizon but it shrinks drastically by the time it’s high overhead. This is purely an optical illusion – the Moon does not change size. You can prove it to yourself by marking with a wax pencil its size on a piece of glass held in front of your eye as it rises tonight. Then a few hours from now hold the glass up to your eye again and compare the size. There is no change!

Looking at the sky is a little bit like having a time machine. Due to the finite speed of light you are not seeing the Moon as it is now, but as it was one and a quarter seconds ago. It takes light 32 minutes tonight to cross the gap between us and Jupiter. If a giant comet hit Jupiter right now, we would not know a thing about it for 32 minutes. The time it takes light to cross a fixed distance is constant so astronomers can speak of Jupiter as being 32 light minutes away in the same way we speak of Naples being a half hour away from Marco. Certainly it’s easier to say than “Jupiter is 371,442,000 miles away tonight.”

Light however must travel years to cover the distance from even the nearest stars. Take Mirphak in Perseus for example; it’s 592 light years away. Putting that in perspective, the light that is striking your eye now left Mirphak in the autumn of 1419 – Henry V was King of England and John II was King of Castile (Spain did not yet exist).

Mirphak is something of an exception, most bright stars are rather closer; Sirius is less than 9 light years off. However it is a very rare star that can be seen with the naked eye more than 1,000 light years off. A large group of stars is a different story; while not visible individually, each star adds its own light to the whole and the group can be visible across vast distances. Take the Great Nebula in Andromeda for example, also known as the Andromeda Galaxy. It’s a huge mass of stars, perhaps 1 trillion of them. Easily visible from a dark sky site with the naked eye, it’s still possible to see with binoculars from suburban skies. The distance? 2,540,000 light years!

See you next time! 

Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples 

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