A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2013 Jul 6, 05:07 -0700
Sean, you wrote:
"The program was discontinued in 1990 and replaced with "MICA" - the Mutiyear Interactive Computer Almanac. (Which is not free.) I have noticed some discrepancies with my printed Naut. Almanac when using ICE, but these have always been within 0.1 arc minute."
The primary data (not the planning data or the less navigationally relevant material) are available on the USNO web site: http://aa.usno.navy.mil/data/docs/celnavtable.php. This site returns data from the same engine as the newer MICA, which was itself just a derivative of ICE (it may well be a local copy of MICA running on their server). If you enter date and position info to match the example you posted, you'll find everything matches up perfectly with the lower half of the table you posted earlier from ICE output, right down to the formatting. In fact, apart from a couple of small layout changes and the inclusion of Polaris in the web version, the only numerical differences that I can see in this case are some small tenth of a minute differences in refraction values at low altitudes. These are of no navigational significance and presumably reflect a small change in the algorithm for approximating refraction (note: this is NOT the same as the very small change to the Nautical Almanac refraction tables that appeared in 2002... or was it 2004?).
You also wrote:
"I'm not sure, but this may have something to do with the apparently incorrect (and unalterable) delta-T value."
Well, let's work this out. The hard-coded value for Delta-T in ICE for today's date as displayed in the output table you posted is 76.3 seconds which is about 9 seconds too high (an early 1990s prediction for the year 2013). This "watch error" (the watch, in this case, being the Earth's rotation itself) enters in as an offset in anything calculated "off-Earth". So the position (SHA and Dec, not GHA and Dec) of the Moon, the fastest moving object, would be wrong by about 9 seconds of time. The Moon moves about half a degree in every hour of time, equal to half a second of arc in every second of time so this would imply a position error of 4.5 seconds of arc or just about a tenth of a minute of arc in the Moon's position relative to other celestial bodies (on the celestial sphere). This would show up primarily as an error in SHA of about 0.1 minutes of arc. We can't tell from the table you posted since the Moon wasn't in the sky and therefore ICE, as well as MICA, omit it from the table. Turning to the other data, you've posted the RA and Dec data for the planets and the Moon, but here you've apparently entered the correct Delta-T value for today since it's showing the time as 00:01:07 TDT, equivalent to 67 seconds Delta-T. Once again, the Moon provides the most sensitive test (this time because of the former difficulty in calculating its coordinates) and its position appears to be off by just about 6 seconds of arc. This difference is not due to Delta-T so it probably reflects a simpler 90s-era method for determining the Moon's position. On this date, the error appears to be in addition to the error that would arise from Delta-T (if the Moon were included in the lower table), but on other dates it would sometimes fortuitously cancel out. From this limited evidence, we could expect that on dates (this year) when the Moon is in the sky, a comparison between the old ICE output and the newer MICA output (from that web calculator) should show differences in GHA for the Moon between 0.0 and 0.2 minutes of arc with occasional 0.1 minute differences in Dec. Positions of other objects should almost always be correct to the nearest tenth of a minute of arc. Plenty good enough for any "normal" celestial navigation!
PS: You said you got this running with DosBox, and I agree that's very useful. For anyone else, if you have old DOS software that you're dying to see come back to life, and as long as it doesn't do much at the "hardware" level, this DOS emulator works really well.
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