Before we answer the question that why v correction is negative for Venus, first let us very briefly recapitulate what is ‘v’ and ‘d’ correction.
‘v’ Correction: It is the actual hourly increase in GHA (Greenwich Hour Angle) above the lowest assumed values of GHA for respective bodies.
Hourly Increase of GHA assumption for various bodies are as follows:
- Sun: 15 Degrees
- Moon: 14 Degrees 19 Minutes
- Planets: 15 Degrees
- Aries: 15 Degrees 02.46 Minutes
Now, if we look at any page of the nautical almanac, for instance as shown in the below images. We will see that there is no ‘v’ correction for Sun or Aries.
The reason for this is that because the average hourly increase in GHA for sun is almost same i.e. 15 deg and hourly increase in GHA for Aries is always same i.e 15 deg 02.46 mins. Hence there is no ‘v’ correction for Sun and Aries.
‘d’ Correction: It is the hourly change in the declination of various celestial bodies.
Now, if we see the above image, we will also notice that there is no ‘d’ correction for Aries.
The reason for this is that first point of Aries is always at the equinoctial with a uniform declination of 0 degrees.
Now, let us come back to our original question, i.e. why v correction is negative for Venus.
It is so because Venus is placed very near to the sun and hence the retrograde motion (apparent direct motion of the Venus) is very rapid because of her proximity (closeness) to the Earth. Hence a minimum value could not be set for hourly change of GHA. Therefore an average value was set and that is why the ‘v’ correction for Venus is sometimes negative because sometimes the actual hourly increase of GHA is below the lowest (average in case of Venus) assumed value.
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Hope, this has given you some clarity on the topic, feel free to comment any doubts or suggestions.