Naked Science Forum

General Science => General Science => Topic started by: chris on 14/12/2004 15:26:41

Title: Why does the earliest sunrise not coincide with the longest day?
Post by: chris on 14/12/2004 15:26:41

Someone asked me a question the other day about solstices and the times of sunsets and sunrises. I admit I didn't have the first clue and would appreciate any help anyone here can offer.

The thrust of the question was "why is it that the earliest sunset and latest sunrise don't actually occur on the shortest day, and vice versa on the longest day ?"

I've not checked an almanac to confirm this observation, but the person asking seemed pretty sure of the observation.

Any clues anyone ?

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Title: Re: Why does the earliest sunrise not coincide with the longest day?
Post by: gsmollin on 15/12/2004 15:03:29

A clue, perhaps. The earth's orbit is not circular, so sunrise and sunset times shift forwards and backwards according to where the earth is in its orbit. The duration of the day changes according the direction of the earth's rotational axis. The solstices occur when the earth's axis is either pointing away from or towards the sun, and those days are the shortest or longest days, respectively. The earliest sunrise may preceed the solstice because the earth is moving towards its perihelion, which occurs on January 12. The same effect occurs for the summer solstice.

I'll see if I can find this in my astronomy references.

Title: Re: Why does the earliest sunrise not coincide with the longest day?
Post by: chris on 15/12/2004 17:15:49

Thanks GS. i got about as far as you in my reasoning, but then lacked the physical knowledge / access to the right information to nail the answer. Intuitively you'd expect the shortest day to coincide with the latest sunrise and earliest sunset, but it doesn't !

Chris

"I never forget a face, but in your case I'll make an exception"
 - Groucho Marx

Title: Re: Why does the earliest sunrise not coincide with the longest day?
Post by: Corbeille on 16/12/2004 15:53:11

Not too sure of the reasons, this website is very good for checking sunrise/sunset times form where you live  
http://www.timeanddate.com/






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