As many of you know, one of my hobbies is celestial navigation. While doing some research on the obscure topic of “v- and d- corrections” I stumbled on this explanation from an AI robot “assistant”.

In the Nautical Almanac, the v correction refers to the correction applied to the observed altitude of a celestial body (like a planet or star) to account for atmospheric refraction. This correction is important because the atmosphere bends the light from celestial objects, causing them to appear higher in the sky than they actually are.

Determining Positive and Negative Values

Positive v Correction: If the observed altitude of the celestial body is higher than its true altitude due to refraction, the v correction is subtracted from the observed altitude.
Negative v Correction: If the observed altitude is lower than the true altitude, the v correction is added to the observed altitude.
The sign of the v correction is determined based on the altitude of the celestial object at the time of observation. Generally, the higher the altitude, the smaller the effect of refraction, and vice versa.

Not Exclusively for Venus

The v correction is not exclusive to Venus; it applies to all celestial bodies observed from Earth. However, the Nautical Almanac provides specific v corrections for various celestial bodies, including Venus, based on their positions and altitudes at the time of observation.

In summary, the v correction is a critical factor in celestial navigation and is used to ensure accurate positioning by compensating for the effects of atmospheric refraction on observed altitudes.

This text is so screwed up its not even wrong.

V and d corrections have nothing to do with correcting sextant altitudes for atmospheric refraction. That correction is called the “Refraction” and is a simple subtraction which is a function of the the observed altitude and (in a few rare situations) extremes in air temperature and atmospheric pressure. The refractive corrections can be quite substantial, especially for objects observed near the horizon. For objects near the observer’s zenith, where the atmosphere is thinnest, the refraction correction approaches zero and can be ignored.

The v and d corrections are applied to the Greenwich Hour Angle (GHA) and Declination (Dec)values for a celestial body derived from the Nautical Almanac. They are NOT applied to the sextant readings, but to the GHA and Dec look-up values, that is, the positions on the celestial sphere of the body at that moment of time. Stars have no v or d corrections, but the planets, sun and moon do, and although they are often tiny and can usually be ignored, in some cases they can be substantial and must be applied to the Almanac values.

GHA and Dec values are due to the rotation of the Earth, which over the course of a year, does NOT vary significantly for maritime navigation applications. However, solar system objects also have their own physical Newtonian motions in the sky, and their positions may need to be updated to reflect these motions. The moon, in particular, has an orbit easily influenced by gravitational effects of other bodies so both v and d corrections are critical. The planet Venus is moving so fast in its orbit, and is so close to Earth, that these corrections, if not applied, can affect a position by several miles. There is no general rule of thumb which can be applied ahead of time to determine if the corrections are really necessary, it must be determined for the time of observation by the navigator’s judgement.

Of course, the fine details of sextant navigation are of little importance today, but it is a topic I am well informed of. I can confidently read the AI generated text and immediately recognize it as totally bogus. However, a student or beginner will not know his v’s and d’s and will be tempted to accept the AI explanation as authentic. And this is the type of error that can be disastrous. It is also not related to the subject matter, it is an artifact of how the AI was “trained”. I cannot trust any AI-generated instructions or analysis because I can see how easily it can be misled–and how authoritative and confident it can sound when dealing with a topic I am not already thoroughly familiar with. The AI got confused reading about one type of correction and conflated it with the other. Its the kind of blunder any student might make…

In this case, it is easy to see how the AI got confused. GHA is the distance in degrees of a celestial body west of the prime meridian; Dec is the distance in degrees north or south of the celestial equator. Refraction error is the correction to be applied to the measured angular distance from the body to the horizon. They have absolutely nothing to do with one another.