The Moon and the planets

The Moon is close enough that its parallax matters: the almanac tabulates its horizontal parallax, and the Moon's altitude correction tables carry parallax, refraction, semi-diameter and augmentation together, by the limb that was touched. A daylight Moon crossed with the Sun gives a fix at once, no run needed. Venus and Jupiter are bright enough for twilight sights and sometimes for daylight; Mars and Saturn at twilight. Their hour angles and declinations are on the daily pages like the Sun's, with a small additional correction for Venus and Mars.

Of the five, the Moon is the most useful and the most demanding.

Why does the Moon need its own correction tables?
Which pair gives a daylight fix without a run?

Show the answers
  1. Why does the Moon need its own correction tables? Its parallax is large, because it is close No other body is near enough for parallax to matter at this precision.
  2. Which pair gives a daylight fix without a run? The Sun and the Moon Two bodies at once give two crossing lines; two Sun sights need the run between them.

The twelve evenings

  1. The celestial sphere
  2. The sextant
  3. Time and the almanac
  4. Correcting the altitude
  5. The noon sight
  6. Latitude by Polaris
  7. The navigational stars
  8. The intercept method
  9. Plotting the position line
  10. The running fix
  11. The Moon and the planets
  12. The day's work

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