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Position of the sun, direction of shadows

Leonard Evens , Jul 04, 2003; 08:33 a.m.

Where can I find information about the position of the sun in the sky as a function of latitude, date, and time of day? I tried a search on "altitude and azimuth", but it referred me to a previous posting that I was denied access to for some reason. There was also a reference to a US Navy site, but I was also denied access to that. I hope I don't have to write my own program to derive the appropriate information. I did find some reference to Palm Pilot programs, but I don't have such a PDA, and I would rather not have to carry one with me in the field anyway.

I understand generally about the path the sun takes in the sky, but I am having a little trouble visualizing it three dimensionally. Can anyone recommend a reference for a refresher course on such matters?

Responses


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charles stobbs , Jul 04, 2003; 08:44 a.m.

See Gallileo

Jeff Schraeder , Jul 04, 2003; 09:20 a.m.

Try looking at a nautical almanac. It will have the altitude and azimith of the sun listed on a minute by minute basis. You need to know your location (latitude) and the time. They are relatively inexpensive but are date specific so you need a new one every year.

Jeff Schraeder , Jul 04, 2003; 09:26 a.m.

http://aa.usno.navy.mil/data/docs/AltAz.html

Try this site. The computed tables give the azimuth and altitude in 10 minute intervals. Negative numbers in the altitude indicate that the sun is below the horizon.

Alex Hawley , Jul 04, 2003; 09:44 a.m.

Leanord, I did a quick Google search for "navigation tables" and found this:

National Imagery and Mapping Agency (NIMA) Maritime Safety Information Division http://pollux.nss.nima.mil/index/index.html

I didn't delve into the site at all but it may get you the information you need for altitude and azimuth of the sun at a specific location.

The standard reference used by the US Navy to explain all this is "American Practical Navigator (Bowditch)". Here's another link that has some of the typical stuff used in celestial calculations and also offers Bowditch for sale.

http://www.waypoints.com/celestialsupplies.html

Calculating sunrise and sunset times by hand is not too difficult. Both the US Navy and Royal Navy publish tables each year to do this. The Local Apparent Noon, the time of day when the sun is at its highest, follows from sunrise and sunset.

The azimuth angle is a bit more difficult and time consuming. Believe me, being able to get this info from the 'net or a computer program is MUCH easier than doing it by hand from the tables. Have fun and good luck!

Kevin Bourque , Jul 04, 2003; 09:45 a.m.

Hi Leonard –

The Naval Observatory publications are helpful if you’re an astronomer, but might be confusing otherwise. If the basic movement of the sun confuses you, then tables of coordinates might not be quite the ticket.

I assume you’re in the Northern Hemisphere. If you face south, the sun will rise on your left and set on your right. It will reach it’s highest at roughly 1:00 PM (not noon, because of daylight savings time). In the summer, the sun rises in the northeast (or slightly behind you if you’re still facing south) and sets in the northwest. In the winter, the sun rises in the southeast and sets in the southwest and makes a lower path in the sky.

From the tropics, the sun rises nearly due east and sets nearly due west and passes high overhead all year long.

From the southern hemisphere the sun moves right to left as you face north.

If this didn’t help, visit your local library and browse through entry level astronomy texts….one will surely have some pictures that will help you. I teach astronomy part time at the local college and I know that visualizing (and explaining!) the movements of the sun and stars is sometimes difficult.

I carry a $3 compass with me so I can predict where the sun and moon will be. It’s helped me many times.

Conrad Hoffman , Jul 04, 2003; 09:50 a.m.

The astronomy folks have lots of programs for this. Do a Google search on something like planetary position and location in sky. Most of the software that does planets also does the sun and moon.

Leonard Evens , Jul 04, 2003; 09:59 a.m.

Charles,

Or I could try Ptolemy instead. He had the geometry wrong, but for calculations his geometry would work just as well as Galileo's, perhaps better.

More seriously, I managed to find some sites myself doing a google search of the net for "position of sun in the sky". One at

www.helioclim.net/com/appletsolargeom/AppletSolar_en.html

has a Java applet which gives a graph showing the altitude as a function of solar time. The altitude and azimuth are given numerically as you move the cursor across the time axis. Unfortunately, with my browser and the current version of Java it is using, some of the fields are overwritten with text, so it is a bit hard to use.

I'm not sure why my original google search yielded so little.

I've also found other sites giving all the relevant formulas, and if I get interested enough, I can write my own software to do what I want.

It would still be nice to see something with detailed three dimensional diagrams showing how the the correct (Coperinican) picture relates to the Ptolemaic picture of the sun moving about the Earth. The latter is what you see when you look up in the sky. I understood that completely at one point in all its subtle detail, but age and time take their toll.

Jeff,

The site you referred me to gives me a firewall error. I am behind a DSL/Cable router which provides a firewall of sorts through network address translation (NAT), but it shouldn't prevent packets from a distant web site from getting through. I think the problem is at the Navy web site. Either it isn't letting any non authorized users through or something about packets coming from a NAT system can't get through. I would be curious to know if others can get through.

Ralph Barker , Jul 04, 2003; 10:09 a.m.

Here are a couple of links that I find handy:

NOAA solor position calculator

and, NOAA Sunrise/Sunset calculator

Rick Jones , Jul 04, 2003; 02:22 p.m.

Redshift is a mutimedia astronomy program available I believe from Amazon and other book sellers. I have version 4. While it provides a lot more information than you may want it will allow you to position yourself anywhere on the planet, pick any date, anytime of day and show you the position of the sun, 250,000 other stars, asteroids, comets and 40,000 deep sky objects. You can place a pointer on the sun or any other object and it will provide exact coordinates. Like I said, maybe a bit of overkill but, who knows, you might find it pretty fascinating as I have.


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