The following information on Star Shots and Ephemeris Data was provided by Mr. Jerry Wahl,, in response to a request for information on computing Ephemeris Data posted on the LSRP Message Board. His two responses are reproduced here with Jerry's approval.

Your all around easiest solution is to get the Elgin*Knowles Ephemeris as it has brief explanations of how to compute an ephem for some stars.

Next to that most survey texts contain a section on astronomic observations and you can purchase the nautical almanac for ephem data.

There are a number of online sites and software programs that can generate ephem data in various forms.

Ephem data that surveyors use is normally tabulated declination and (Greenwich Hour Angle) GHA for the particular star. The GHA can be derived from it's SHA (Sidereal Hour Angle) or Right Ascension (RA) and the SHA of Aries. This form has the advantage that a star's coordinates in the form of declination and SHA or RA changes very little and can be tabulated only once a month or so. So the exact procedure to follow in computation initially will depend on what data you get.

There are two or three observation methods similar to Solar observations. Polaris is almost unique in that being close to the pole it moves very slowly and is thus much more immune to errors in time and position of the observer. Other stars move rapidly and observing them is much like doing a Solar except the pointing is to a point light source. At night this requires some technique to handle cross hair and scale illumination.

Observation on a non polar star will have some of the same general sources of error as a Solar. If you do an hour angle observation you will need to know your your longitude and time especially accurately. Pointing may be easier and more accurate than on a limb of the sun, but more skill is required to find, see and track a star. A star can also be computed with the altitude method which is less critical for time and longitude error and the primary error budget shifts to derivation of the refraction correction which then requires at a minimum temperature and pressure and still may only be derivable to 10 seconds or so.

Thus I think a non polar star is probably capable of the same accuracy as a Solar, which for an hour angle observation depends on special care in time keeping may get to less than 6 seconds with reasonable care.

Polaris on the other hand can probably be less than a second. For Polaris the biggest problem is the ability to level and instrument errors. The position of the star itself can be accurately determined sub second easily with time to within 5 or 10 seconds.

I would have to do some quick research on specific net sites for ephem data, so if you follow up I can come up with a few in addition to software which can be used to generate your own ephem data.

Since the formula are in most texts I will not repeat here unless needed in a subsequent message.

Software you can download.

ICE or the Integrated Computer Ephemeris. This programs was originally issued as a beta by the Nautical Almanac Office and is a multi year, very accurate ephemeris generator. This was replaced with a very similar program called MICA which is now sold. However ICE is still hanging around and gives darned near the same answers. Information can be found here:

USNO main site

ICE explanation

Here is one site that has it for download:

Ice Download site

Online interactive sites and information:

It should be noted also that there is a ton of freeware and shareware programs for astronomers which plot the sky and generate ephemeris data. Some of these are very accurate, I would suspect that some are not. We have used the demo program skymap pro and it seems to generate accurate data, for example.

USNO has MICA online so you don't have to buy it, and it can be used to check ICE. There is one subtle difference between them which has a small effect on generating data for surveyor's use. MICA is more directly useable.


International Astro

NASA JPL's generator

of course there is my Solar and Polaris ephem at Jerry's but this doesn't cover any other stars.

Someone elses list of ephem sites:

High Precision Ephemerides

Solar only?

Solar data

Now it would take some time to outline how to use each of these to generate useful information of the form Surveyors use. I will try to outline using the online MICA site in a subsequent message....

Thanks Jerry, good stuff!
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