Hi all, a ton of great info here.

I'm curious for any of you skilled in celestial navigation. I just purchased a fun new toy, a solunar watch from Sky Time. It's a fun outdoor watch as it essentially has a 24 hour dial with a single hand which, by shading the disk and the outside ring, shows graphically sunset, sunrise, moonset, moonrise, and the phase of the moon.

One of the neat features also, though is that it will give altitude and azimuth of the sun at any given time during the day. The azimuth of the sun would give me a way to find true north on a sunny day, and I think makes a nice backup for my compass.

Sunrise and sunset information, as well as moon phase, rise and set obviously also would be useful in a survival situation.

Any ideas on what other uses having access to this type of info could provide? The watch allows you to set your location by city or by long/lat to make its calculations. One idea I had is that by, say, comparing the azimuth for a known location, to what I observe I might be able to figure out a rough location if I really didn't know where I was?

I bought this primarily because I find the sunrise/sunset and moonrise/phase/moonset useful, but I'm also finding it quite fun to play with.

For those who might be interested, there is more on the watch here:

http://www.skytimeonline.com/default.asp

(note: I'm not sure if it is OK to post links, mods, feel free to remove if this isn't kosher, believe me, I'm not shilling for the company, nothing to do with them at all)

Yes watches make a similar timepiece

http://www.yeswatch.com/

but I'm unsure if they give azimuth info.

Nice to meet you all.

T.B. Stevens

Welcome here T. B..
Altough this watch packs a bit more computing power than I´ d want on my wrist it may be a nice tool.
For the rough estimation of your position you also need the altitude of the sun to determine the distance. When you make two observation a least one hour appart and do some correction for the refraction the position could be in a 5 sm radius of the estimation. I tried that with a cheap cardboard sextant and it was better than that most of the time. When you search for protractor or sextant you may find some information about the additional tools and techniques involved.
The links should be OK as others post links too. After all we might wish to know where we could get that gear.

I'm not sure how much "homework" this device does before churning out the altitude and azimuth of the sun, and this will affect its accuracy greatly.

Also, I don't know how familiar you are with the motions of the sky, so please forgive me if this is too elementary.


The altitude and azimuth of the sun depends on three things: The current local time zone (i.e. EST, CST, PST, etc.) time (and whether daylight savings is applied or not), the calendar date, and your location (latitude and longitude).

The sun moves in the sky 15 degrees per hour, or one degree every 4 minutes; it traverses the diameter of its own 1/2 degree disc in about 2 minutes.

The date and latitude determine the culmination angle of the sun, i.e. how high it gets at local noon. At culmination, the sun is ALWAYS exactly due south (in the northern hemisphere, or due north in the southern hemisphere). There's the usual seasonal variation plus a factor depending on your latitude. The fomula is:

Culmination angle = 90 - latitude + seasonal_angle

where seasonal_angle represents the perpendicular component of the earth's axial tilt; this is +23.5 degrees in summer (June in the northern hemisphere, December in the southern), -23.5 degrees in winter (the obverse of above), zero at the vernal and autumnal equinoxes, and somewhere in between for all other dates. If the culmination angle is greater than 90 degrees (and this only happens in the tropics, defined as latitudes less than 23.5 degrees), this means the sun culminates on the "wrong" side of the sky - 105 means the sun is 75 degrees high toward the NORTH in the northern hemisphere, etc.

But while the sun is always due south (or north, in the southern hemisphere) when it is at its culmination, exactly WHEN this happensisn't so clear - nominally it's "high noon" but it can vary from this by nearly 2 hours in some cases!

First, there's daylight savings - if you're using it (these parts it's April thru October), subtract 1 hour from whatever time the clock reads. So far so good.

But even then you're not done. Recall that your time zone time is not your local time at all, but rather the time at some arbitrarily defined line of longitude, and applied uniformly throughout the "zone" over which this time applied; this is the legacy of the need for area scheduling for railroads in the 19th century. These "zones" are about 15 degrees (of longitude) wide and are nominally centered at longitude lines of 0, 15, 30, 45, 60, 75, 90, 105, and so on - if your longitude differs appreciably from those "standard" longitudes for the time zone you're in, the time of mean solar transit will differ in minutes by four times the longitude difference in degrees (i.e. if you're at longitude 71 west, you're in the Eastern Standard Time zone, centered at 75 west, so the mean sun transits 16 minutes before noon). Nominally, this means that the mean sun transits up to half an hour before or after zone time noon, but because political and other considerations factor into zone delineation, the difference can be somewhat greater than that as well.

In North America, the time zones are centered as follows:

Greenland/eastern South America (-3): 45 W
Newfoundland (-3.5): 52.5 W (this is one of the unusual "half hour" zones, the largest worldwide being India's +5.5 zone)
Atlantic (-4): 60 W
Eastern (-5): 75 W
Central (-6): 90 W
Mountain (-7): 105 W
Pacific (-8): 120 W
Alaska (-9): 135 W
Hawaii (-10): 150 W

And notice I kept saying "mean sun." In the desert the sun can be mean, but that's not what I'm getting at. Because the earth travels in an elliptical orbit, with its distance from the sun and orbital speed varying periodically over the course of the year, the length of a solar day (defined as the period from one local solar noon to the next) can vary by as much as a few seconds over the course of the year. Because these deviations are cumulative, even after correcting for daylight savings and for longitude, the "real" sun can transit as much as 22 minutes earlier or later than 12 noon. This corresponds to a deviation in the sun's position of 22 minutes*1/4 degree per minute = 5.5 degrees east or west. To correct for this you apply something called the "equation of time," tables for which can be found in any good book on navigation and man web sites or books on astronomy.

Once you correct for daylight savings, latitude and longitude, seasonal variation in the sun's declination, and the equation of time, you can get the sun's position to an accuracy of 1 degree per four minutes of error in your time (or the time to within four minutes for every degree of positional error). If you do not apply those corrections (other than daylight savings, which is trivial), you can find the sun's position from the time to within about 10-15 degrees most of the time, and you can get the time from the sun's position to within an hour or so.

While the latter sounds pretty bad, bear in mind that unless you correct for magnetic declination (maps are available for this purpose), in most of the 48 states, since the magnetic north pole does not coincide with the geographic north pole, but (currently) lies near Bathhurst Island in northern Canada, the compass misses geographic south by up to 20 degrees, although usually the deviation is more usually 5 degrees (in the south and near the continental divide) to 10-15 degrees (in the northeast and northwest). In Canada and Alaska, the deviations can be greater. Australia is similarly afflicted, since magnetic south lies somewhere in the ocean, not at the geographic south pole. In Europe, Africa, and most of Asia the results are better - the deviations are usually within about 5 degrees or so, and rarely exceed 10, except at high latitudes.

I'll stop for now.