The celestial sphere

Imagine every star, the Sun, the Moon and the planets fixed on the inside of a vast sphere centred on the Earth. A body's place on it has two coordinates, as a place on Earth has: its declination, north or south of the celestial equator, is the sky's latitude; its hour angle, measured westward from a meridian, is the sky's longitude. The almanac gives the Greenwich hour angle and the declination of the Sun, the Moon and the planets for every hour, and the sidereal hour angle of the stars, which is added to the hour angle of Aries.

The geographical position of a body is the point on Earth directly beneath it: its latitude is the declination, its longitude the Greenwich hour angle, west. Everything in celestial navigation comes back to that point: a sight measures how far you stand from it.

A body's declination corresponds to which coordinate of its geographical position?
Which quantities does the almanac tabulate for a star?

Show the answers
  1. A body's declination corresponds to which coordinate of its geographical position? Its latitude Declination is measured from the celestial equator as latitude is from the Earth's; the hour angle plays longitude's part.
  2. Which quantities does the almanac tabulate for a star? Its declination, Its sidereal hour angle A star's sidereal hour angle and declination change slowly; its Greenwich hour angle is the hour angle of Aries plus the sidereal hour angle.

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

A demonstration: the course, its tutor and its loft are invented; the navigation is real. Built by plinth from EDN and Markdown; the quizzes are checked in the page and send nothing, and the only thing kept is your progress, in your own browser.

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