Aries, the Greenwich of the celestial sphere

Just as on Earth, a reference point has been chosen on the celestial sphere through which the zero meridian passes. Since we do not yet know any small towns on the celestial sphere, something else had to be selected as the equivalent of Greenwich. The chosen point is the vernal equinox. This is the point in the sky where the Sun is located in spring when it is directly above the equator. The Latin name of this point is Aries.

Aries is of course also a zodiac sign (the Ram), but that is not important in this context.

The technical explanation is not really important. What matters is knowing that such a precisely defined point on the celestial sphere, Aries, exists. Its declination is, of course, 0°, and the celestial prime meridian passes through this point and the celestial north pole above the Earth's North Pole.

Aries is denoted by the symbol .

C o n t e n t s

• Position fixing using stars
• The night sky
• The celestial sphere
• What you see from the North Pole as the Earth rotates
• How the starry sky changes when traveling to the equator
• Effect of Earth's rotation on the sky at the equator
• Celestial meridian, upper culmination and lower culmination
• Change in star altitude at the North Pole, equator, and between them
• Declination
• Example of determining latitude
• Hour angle, Greenwich Hour Angle (GHA)
• Aries, the Greenwich of the celestial sphere
• Sidereal hour angle (SHA)
• Local hour angle (LHA)
• Relation between hour angle and time
• Example of determining longitude and latitude from the Sun at noon
• Relationship between angles, degrees and nautical miles on Earth’s surface
• Position circle
• Azimuth (A, Azimuth)
• True bearing to a star
• Nautical triangle
• Solving the nautical triangle (altitude calculation)
• Celestial navigation using Marcq Saint Hilaire intercept method
• Star identification
• Details and practice
• Time
• Time difference (ΔT)
• Altitude measurement
• Example of position fixing
• Twilight
• Calculating twilight time
• Navigation Association exam