Sidereal Compass

• The Sidereal Compass means navigating by the stars, instead of using a magnetic compass. Sidereal is from the Latin Sidus meaning star.
• The Sidereal Compass rose has compass points that indicate the rising stars on one side and the setting stars on the other.

The Solar Day and the Sidereal Day

• • The Solar day
    • is measured from Noon to Noon, when the Sun is directly overhead,
  • The Sidereal Day
    • is measured from one complete rotation of the Earth on its axis. But the Earth has moved around the Sun by approximately one degree in its orbit. So the Earth needs to rotate a little more for the Sun to be directly overhead at Noon.
  • This difference is 4 minutes longer each day.
  • Therefore, the stars rise and set 4 minutes earlier each day.
  • The Solar year
    • there are 365 1/4 Solar days, but 366 1/4 Sidereal days (4 minutes x 365 = 24 hours).
    • after 365 solar days, the stars will rise and set at exactly the same time, and the cycle starts again.

Sidereal Time

• On Earth, a terrestrial position is obtained by its Latitude and Longitude.
• In the sky, a celestial object is measured by the equivalent of Latitude and Longitude:

• • Its Latitude
    • is called Declination, which is its Latitude measured from the Celestial Equator.
  • Its Longitude
    • is called Right ascension, measured in Hours, Minutes and Seconds, from a zero point on the Equator, known as the First Point of Aries. This is where the Sun crosses the Celestial Equator at the Vernal Equinox.
  • Sidereal Hour Angle (SHA)
    • which is similar to Right ascension, is used in Celestial Navigation but increases Westwards.

The Wind Compass

• The Wind Compass marked the various cardinal points and their divisions by the winds that blew from that direction.
• Greek and Roman navigators used the wind compass when cloud obscured the stars.