Time Difference (ΔT)

In navigation, time difference refers to the difference in degrees between the longitudes of two locations, and the time the Sun takes to move from the meridian of the first location to the meridian of the second. The time difference (ΔT) between two locations does not refer to time zone differences. The zone time difference between Helsinki and Stockholm is 1 hour, but the actual time difference is only 27 minutes 33.6 seconds.

Helsinki Central Railway Station is located at longitude E 24° 56.5' and Stockholm Central Railway Station at longitude E 18° 3.1'. The difference in longitude is 6° 53.4'. Since the Sun moves 15° per hour, the Sun’s projection point travels from the longitude of Helsinki to the longitude of Stockholm in 27 minutes 33.6 seconds. Therefore, the time difference between Helsinki and Stockholm railway stations is 27 minutes 33.6 seconds.

Apperent time in Helsinki is 12:00:00 when the Sun passes the Helsinki meridian at E 24° 56.5'. At the same time, the apparent time at Stockholm’s meridian E 18° 3.1' is 11:32:26. Similarly, in Stockholm the apparent time is 12:00 when the Sun crosses the Stockholm meridian. At that same moment, the apparent time in Helsinki is 12:27:34. The time difference is therefore also the difference between the apparent and mean times of Helsinki and Stockholm. In other words, the time difference between Helsinki and Stockholm is the time it takes for the Sun’s projection point to move between the two meridians. This was discussed in the chapter Relationship between hour angle and time.

It can also be said that the time difference is the hour angle difference between two locations.

C o n t e n t s

• Position fixing from stars
• The night sky
• The celestial sphere
• What you see at the North Pole as Earth rotates
• How the night sky changes when traveling to the equator
• Effect of Earth's rotation on the sky at the equator
• Celestial meridian, upper and lower culmination
• Change in star altitude at pole, equator and between
• Declination
• Example of latitude determination
• Hour angle, Greenwich Hour Angle (GHA)
• Aries, celestial Greenwich
• Sidereal Hour Angle (SHA)
• Local Hour Angle (LHA)
• Relationship between hour angle and time
• Example of longitude and latitude from the Sun at noon
• Angles, degrees and nautical miles on Earth
• Position circle
• Azimuth (A)
• True bearing to a star
• Nautical triangle
• Solving the nautical triangle (sight reduction)
• Position fixing 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 exam