How accurate are sunrise/sunset times?

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How accurate are sunrise/sunset times?

Postby ed_b » Tue Aug 29, 2017 12:45 pm

Hi all,

I'm new here and new to astronomy, and have a basic question - so this looks like a good forum!

I'm using the outputs from HMNAO Websurf for sunrise and sunset, and wondered if anyone can tell me how accurate these are?

In particular for sunrise/set, do they account for refraction of the earth's atmosphere, which makes the sun appear to be above the horizon even when it is geometrically below it?

And generally how accurate are these times? There's what seems to be a US Gov site, which suggests that even when multiple variables are entered, the output can only be within ±30s.
https://midcdmz.nrel.gov/solpos/spa.html

Any ideas? If this is one for HMNAO I can ask them, but wondered if there's a sort of industry standard that means it's too obvious a question.

Thanks all,

Ed B
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Re: How accurate are sunrise/sunset times?

Postby Gfamily2 » Tue Aug 29, 2017 12:59 pm

ed_b wrote:Hi all,

I'm new here and new to astronomy, and have a basic question - so this looks like a good forum!

I'm using the outputs from HMNAO Websurf for sunrise and sunset, and wondered if anyone can tell me how accurate these are?

In particular for sunrise/set, do they account for refraction of the earth's atmosphere, which makes the sun appear to be above the horizon even when it is geometrically below it?

And generally how accurate are these times? There's what seems to be a US Gov site, which suggests that even when multiple variables are entered, the output can only be within ±30s.
https://midcdmz.nrel.gov/solpos/spa.html

Any ideas? If this is one for HMNAO I can ask them, but wondered if there's a sort of industry standard that means it's too obvious a question.

Thanks all,

Ed B

Hi and welcome.
The fact that they are asking for the air temperature and pressure is a strong indication that they are applying a refraction adjustment, and I suspect that is the cause of the uncertainty for the calculated.
If you're interested, it would be worth making adjustments to the temp and pressure while keeping everything else the same, you can then see how it affects the setting time.
Bear in mind that the refraction depends on the totality of the atmosphere it passes through, so although it asks for your local temp and pressure, you can't get a precise value based just on the local values.
Scopes: Meade 8" SCT, Skywatcher 127mm Mak
For imaging: Pentax K5, Asda webcam, Star Adventurer (new toy)
For companionship: Mid Cheshire Astronomical Group.
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