Saturday, December 4, 2021

It’s about time!

Looking South, 9:00PM, September 30th.

Looking South, 9:00PM, September 30th.


By Mike P. Usher 

This particular Autumn is a bad time for early evening planet watchers; bright naked eye planets seem to be missing from the sky. It will be midnight before Jupiter will be high enough for good viewing, Saturn is behind the Sun, and it will be December before Venus will be a decent evening star. It will be spring before Mars and Mercury will be visible at a reasonable hour. At least the glorious summer Milky Way is still visible!

You are undoubtedly aware the movement of the sky is a very regular affair; until the mid-twentieth century the sky was the ultimate standard to which earthly timepieces were set. But it is also very easy to understand and make allowances for. Did you know that you can keep tonight’s chart (and all of our published charts too) and use it at anytime of the year by making a simple time correction?

The accompanying chart shows the Milky Way in the south at 9:00PM on September 30. The rule is you add two hours for each month previous to the listed date and subtract two hours for each month subsequent to the listed date. The sky will look precisely the same at 7:00PM October 30th and was the same at 11:00PM August 30th. If you wish to look at the sky on May 31st using this chart, calculate 4 months x 2 hours = 8 hour time difference, add it to the listed 9:00PM time and the sky will look just like this at 5:00AM. With just slightly less accuracy you can use 1 hour for a two week time period or one-half hour for a week.

Note that 12 months x 2 hours = a 24 hour time correction so next September 30 at 9:00PM the sky looks just like it does tonight! The nice round numbers are no coincidence of course – the ancients who invented timekeeping and our calendar based both on the sky and created convenient units to match.

The major exceptions to the two hour per month rule are Solar System objects like the Moon and planets – they move around! The very word “planet” is derived from the ancient Greek word for “wanderer.” Venus and Mercury can move noticeably in a single day while slowpoke Saturn might spend two years in a constellation. The Moon moves its own diameter in a single hour. If you are using an old chart and you see a bright star not on it, it’s a safe bet it’s a planet.

If any reader has a topic he or she wants covered, please drop me an e-mail!

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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