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Re: Estimating horizon from a photo on a clear night
From: Frank Reed
Date: 2024 Dec 15, 09:47 -0800

Bill L., you wrote:
"Just an example. Suppose we can see on average one star per square degree, and our super horizon camera can see 360 degrees of horizon and one degree of altitude. We would have around 360 stars. If they were spread evenly in altitude we would have one for every 1/6 of a minute. If we took the lowest 3 stars to define the horizon we might expect to have the horizon to around 1/2 a minute. "

That's not going to work because of common (normal) atmospheric extinction. Here's an old but useful link: ICQ Magnitude Corrections for Atmospheric Extinction. Like refraction (and mathematically similar, too), extinction is highly non-linear near the horizon. In the linked article, look at the extinction values at sea level near the horizon in the tables at the end. Under "average" conditions, the extinction for an observer at sea level at 1° altitude above the horizon is over 7 magnitudes. This means that the brightest true stars (e.g. Arcturus, Vega, etc.) which have zenith magnitudes near zero would all be knocked down to magnitude 7 when so close to the horizon, according to the tables. In my experience, that number is too pessimistic and 3-5 mag of extinction is more reasonable, but then again at such low altitudes and with such great variability close to the horizon, it does depend critically on how we define things. In any case, even the brightest stars will be significantly muted and possibly only visible in binoculars. The planets Venus and Jupiter can be seen at very low altitudes below one degree and the Moon, too. That's about it.

Stellarium simulates refraction and extinction in its normal behavior. You can turn the atmosphere off to compare. In the attached images, you can see how the altitude and magnitude of the star "Regor" (gamma Velorum *) changes with and without atmosphere. It's a good match for what I have seen visually, and it appears that the extinction in the simulation is, in fact, more optimistic than the values in the tables linked above.

Frank Reed

* Some long-winded, rambling background on star name trivia ... since this is an issue that often comes up among Stellarium users. The star known since the 19th century as "gamma Velorum" (originally in the constellation Argo Navis before it was split up) was named "Regor" during the Apollo missions in the 1960s/70s, and it was apparently listed that way in the tech documentation and manual for the navigation computer. That name started as a prank, and as is now widely known, Regor is Roger --reversed-- and was named for Roger Chaffee. The other names listed, as in the attached images, are a mixture of useful and nearly pointless. The potentially useful "names" are cross-listings in various prominent star catalogs. The "HR" id is the Yale-maintained star catalog, known as the "Harvard Revised" Bright Star Catalog id. The HRBSC is a key list of approximately 9100 stars --basically, like the title says, the "bright stars". "HD" is the Henry Draper catalog of stars, and "SAO" is Smithsonian Astro... catalog, while "HIP" is the id in the 1990s Hipparcos catalog. After those useful cross-ref ids, there are double star identifications which are not mostly necessary or useful. For example "WDS" is the Washington Double Star catalog id, maintained in part by the ths USNO (Naval Observatory), which looks mysterious (J08095-4720) but is actually little more than a repetition of the standard (epoch 2000) coordinates. The "J" is merely a reminder that this id is "J2000" while the numbers are just the RA and Dec of the star in traditional hours and minutes and degrees and minutes respectively. Looking at the star's Dec in the screen capture, it's listed as "-47.3338" which in degrees and minutes is near enough to -47°20' and, sure enough, that's the second half of the WDS id (and yes, the double star experts are aware that this identification system is doomed). In addition, this star is identified by two distinct double star "discoverer IDs", namely "Δ 65" and "TOK 2". The first is a legacy shorthand and can also be written "DUN 65" (Δ=DUN or Dunlop, Σ=STF or Struve, and some others). The "alpha" codes, like DUN instead of Δ, were said to be "modern" and the Greek labels "obsolete" back in the 1960s, but that was mostly because there were no available Greek letters for punched cards at that time. The Greek IDs are actually still popular in the narrow field of double star observers, but what they tell us is fundamentally trivial. The discoverer was really usually the cataloguer, and DUN was an astronomer named "Dunlop" but it's not as if he actually "discovered" this star. He merely catalogued it and measured its position angle and separation. The second ID is a continuation of that tradition in the 21st century. The label "TOK" here is short for Tokovinin, who continues the long tradition of double star/multiple star system astronomy even today. Thus "TOK 2" refers to a companion to Regor, maybe physical, maybe not. Want more identifications and catalog numbers? Visit SIMBAD for this star and scroll down to "Identifiers"! Crossing threads, the star known to navigators as "Rigil Kentaurus" probably more commonly known as "α Centauri" is listed in the double star "discoverer" codes as "RHD 1". The RHD refers to a late 17th century French Jesuit observer, Jean Richaud, who was at that time in Pondicherry, India (a French colonial holding for centuries) and who also was involved in the determination of the longitude of Pondicherry by lunar eclipses or by eclipses of the Galilean satellites of Jupiter which is listed on the Cassini global map we were talking about recently. Richaud was apparently the first person to point a telescope at alpha Centauri, and of course it was immediately obvious that it was a double star. :)

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