Celestial Ephemerides Options for Sun and Polaris

I tried a different example, from the online content that accompanies Ghilani's "Elementary Surveying: An Introduction to Geomantics"; here's the data

--------------------------------------Sun

Solar reduction using data from Example 18.17, June 29, 1988

Observer's Astronomic Position:
Latitude = 42°45'10.0"
Longitude = 73°56'30.0"

StopWatch Start Time, UTC: 0:00:00.0
DUT correction: 0.0sec

GHA of Star at 0h UT : 179°09'25.50"
GHA of Star at 24h UT : 179°06'27.40"

Declination of Star at 0h UT : 23°13'55.30"
Declination of Star at 24h UT : 23°10'30.00"

Time: 20:53:010 UTC

Azimuth to star 267°16'21"

(Only relevant parts of example included.)

Using the same navy.mil website as above, but choosing "Topocentric Positions of Major Solar System Objects and Bright Stars", I entered the above info and obtained the azimuth to the Sun as 267°16'22.1". I'm no expert on this stuff, but am I right in saying that the azimuth to the star could easily be adjusted by merely adding or subtracting the semi-diameter correction = Sun's semi-diameter/cos h where h is the angle above the horizon, depending on which edge of the Sun had been sighted? Wouldn't this skip many steps?

Keep up the good work Shawn. Naval Observatory has lots of good information. EOP etc . I have been there several times and was lucky enough to get to spend some time in the library and found that I realize just how un-educated I really am. Also how brilliant some of those people were to figure out so much and give us the mathematic foundation that we still have today in order to do all of what you just did. I mean they didn’t have computers or cesium or rubidium atomic frequency standards AKA atomic clocks. Pretty amazing how those who were stationed there and all over the world and had snail mail to get messages from one observatory to another by carriers from walking to horse boat so that they could do computations and probe and disprove theories. Now we can push a button and achieve results in mere seconds that if it were not for those before us that had the discipline and character and devotion. When you think about it a man lived through all weather conditions no grocery store AC unit or heat pump. And kept records that has set a foundation for our current Datums both locally and globally. You keep inspiring as this is the stuff that those up and comers need to see. Might be one day we will have to resort back is some huge issue happens and all the satellites get zapped by some huge solar or space activity.

I notice Jerry Wahl's data is necessarily geocentric, since there is no opportunity to put in the observer's latitude and longitude. The USNO and Horizons data is topocentric, that is, from the observer's point of view. When giving the observer's latitude, longitude, and elevation, the elevation is measured from the ellipsoid, not from the center of the Earth. So giving an elevation of 0 is not the same as a geocentric R.A. and declination.

Great find, Ashton. I was so focused on getting GHA and the right settings to get the same ephemeris data that I know works for the surveying programs I've used in the past for Azimuth determination that I didn't even notice that.

From a production standpoint, that would definitely be the way to go. I'm wanting to dig into geodetic surveying and field astronomy for my own personal interests, so I'm more interested in the components than just the final answer, but again, thanks for pointing that out.

Yes. This was one of the main points of my post was trying to find what combination of settings are appropriate for getting the ephemeris data that would work in surveying software programs I've used in the past. Topocentric vs. Geocentric was a big one.

I notice that the equations in Gilani's text make no distinction between a star, the Sun, or the Moon, except for semi-diameter. But the Astronomical Almanac for the year 2011 (pp. B84-B86) indicates a simple correction for parallax and aberration needs to be made to the R.A. and declination of the Sun because the observer is not at the geocenter; simple formulas are given. It says more elaborate corrections need to be made for the Moon because it is closer, and no formulas are provided. It appears the maximum parallax is about 8 arcseconds.

I found that also in the text I'm working from "Plane and geodetic surveying for engineers" by David Clark Vol. 2 Higher Surveying.

It gives the formula for geocentric parallax as GC= 9"*cos(h') where h' is the observed zenith angle to the Sun. "This is to be added to the observed altitude".

I played with the stated value of 9" and noticed that it corresponds to the Earth Radius (~4,000 miles) and the Distance to the Sun (93,000,000 miles). Arcsin 4,000/93,000,000 = 9"

So for the moon, I would guess the same formula would apply but use the Earth Radius (~4,000 miles) and the Distance to the Moon (226,000 miles). Arcsin 4,000/226,000 = 1°00'51" = 3,651"

The part about the moon is all conjecture on my part and could be entirely wrong.

As you play around. Polaris and the sun are not the only stars you can utilize. The sky is full of many stars that one can use. I am heading out of town in the morning for a week. But maybe i will remember upon my return and find my old notes and star charts. I imagine NGS still has a lot of information tucked away somewhere. The late Dave L did a class at Corbin I remember 1 around 2009 he conducted. I was living on the grounds at the time. He had the class randomly pick different stars and had some goggle looking things that identified them and then it was all fun time.

Now one thing that you must have is the correct amount of observation fluid. If that is not utilized the azimuth is just a guess lol. Just kidding but good old Dave D would probably say i was correct.

One possible reason surveyors might be looking through this site for astronomical azimuth information is to prepare for the NCEES surveying exam. I won't be taking that, but I looked through the free booklets (FS & PS) that can be downloaded from their site; I understand a copy of the booklet will be made available while taking the test. I didn't see anything about astronomical azimuths.

Also, I looked for the Astronomical Almanac for the year 2024 at the Government Publications Office and Amazon; it wasn't available yet. Apparently you can get it from some obscure maritime suppliers; maybe they're getting it from His Majesty's Nautical Almanac Office (HMNAO). But the fact that the book isn't readily available this late in the year indicates to me it is no longer used for day-to-day work and is only a reference book. Looks like the HMNAO's Almanac for Land Surveyors is the only remaining published almanac that can be obtained on a timely basis.

My take on this is astronomical azimuths have disappeared from the repertoire of most surveyors and has become a niche skill. But let me know if I've got this wrong.

I have done a lot of astro obs (mostly 30 to 40 years ago) using a T3 and a T2, and have used Jupiter, solar, polaris, the moon, and other stars.

When I was a student at Purdue, I suggested to my geodesy professor that we should be able to use the trailing edge of the moon, just like we do for solars. He didn't think it was possible, said that the orbit was not accurate enough. I felt it should be accurate enough using the right data, as they have had retroreflectors on the moon since the Apollo missions. I proved it was possible, using the polynomial coefficients available from the astronomical almanac. However, these polynomials are no longer provided by the USNO. It is imperative to use topocentric coordinates rather than geocentric due to the "closeness" of the moon as compared to the radius of the earth.

I did find a web page by a fellow named George Gladfelter where he has calculated these polynomials up until 2101 (item 10). Lots of other interesting stuff on this page...

http://ggladfelter.net/Astro/almanac.htm

I have downloaded the NOVAS software package https://aa.usno.navy.mil/software/novas_info (I downloaded the C version). It took some doing to figure out on a windows computer with the command prompt and Visual Studio, but I worked it out. It would be used by writing a short C program, and reading in a simple file of your own design that gives delta T, the difference between UT1 and UTC, the time of the observation, and the celestial body you want. The body could be the Sun, any planet, the Moon, or a wide selection of stars. In a few lines you can get the RA and declination of the body, and its azimuth and altitude. I've attached the output file for the example that comes with the software.

A few advantages are (1) it is self-contained, no need for an internet connection and (2) it runs fast; I wouldn't be able to time it with a stopwatch, I'd have to build timers into the program.

You are correct in that almost no one is doing astro obs for azimuth nowadays (other than for fun 😁), but if the GNSS systems ever go down...it will become a valuable skill

While not at all likely to happen, it could happen due to a massive coronal ejection or a war which escalates...

A mundane reason GNSS could become unusable for some people would be GPS jamming by truck drivers who don't want their bosses to know where they are, which only affects the vicinity of the truck. Or, a government shutdown could make OPUS unavailable, so those accustomed to that work flow would have trouble.

I have ephemeris files through 2015-2027 in a text file format - now posted here Document->Public folders>ehpemerisKBW. (sorry if you already saw this - I previously didn't reply in the thread)