Time and the almanac

Longitude is time. The Earth turns fifteen degrees of longitude in an hour, one minute of arc in four seconds, so a watch four seconds wrong moves a position line a nautical mile east or west. Keep the watch to Universal Time from a radio or satellite time signal and note its error and its rate.

The daily pages of the nautical almanac give, for every whole hour of Universal Time, the Greenwich hour angle and the declination of the Sun, the Moon and the four navigational planets, the hour angle of Aries, and the Moon's horizontal parallax; the increments and corrections at the back carry the hour forward to the minute and second of the sight. Read the day, the hour, then the increment: two numbers added, and the body's place on the sphere is yours.

An error of four seconds in the time of a sight moves the position line by about how much?
Which do the almanac's daily pages give for every hour?

Show the answers
  1. An error of four seconds in the time of a sight moves the position line by about how much? One nautical mile Fifteen degrees an hour is one minute of arc in four seconds, and a minute of arc is a nautical mile.
  2. Which do the almanac's daily pages give for every hour? Their declination, The Greenwich hour angle of the bodies The altitude is yours to measure; the almanac places the body.

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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