Precession of the Equinox

1. Precession of the Equinox • The Earth’s axis changes direction in a clockwise direction with a cycle of about 26,000 years

2. Precession of the Equinox 2 • The precession is caused by the Earth’s equatorial bulge (the Earth is not a perfect sphere) • The longitude of the VE changes by 1°about every 72 years • It can be discovered by careful analysis of old star charts

3. Precession of the Equinox 3 • Discovered by Hipparchus around 150 B.C.E. • Discovered independently in China around 320 C.E. by Yú Xǐ (虞喜) • These are the only known discoveries of precession • Don’t confuse it with “procession”

4. Precession of the Equinox 4 • The equinox moves clockwise along the ecliptic • The NCP traces out a circle in the sky

5. Precession of the Equinox 5

6. Precession of the Equinox 6 • Most of the time there will not be a good pole star • We are lucky to have Polaris in our age

7. Solar and Sidereal Day • Sidereal day is one rotation of the Earth or the day with the respect to the stars and is about 4 min shorter than the solar day • 24 h/360 = 24*60 min/360 = 4 min • A year of about 365 solar days has about 366 sidereal days

8. Solar and Sidereal Day 2

9. Tropical and Sidereal Year • The tropical year of a bit less than 365.25 days is (to a first approximation) the time from one vernal equinox to the next and corresponds to the seasons • Because of the precession of the equinox, this is NOT the same as the sidereal year of a bit more than 365.25 days , which is one revolution in the orbit or the year with respect to the stars

10. Time Zones • The time at Greenwich is referred to as UTC (Coordinated Universal Time), but was earlier called GMT (Greenwich Mean Time) • Each 15° (360/24) east or west of Greenwich corresponds to one hour earlier or later

11. Time Zones 2

12. Time Zones 3 • We are at longitude 104°, so we would expect to be in UTC+7, but in fact we are in UTC+8 • We are on permanent Daylight Saving Time • Time zones tend to drift westward • Russia has 11 time zones! • China is the largest country with only one time zone

13. Time Zones 4 • Before 1949, China had five time zones

14. Time Zones 5 • China could not have enforced a “one China” policy on time zones if the capital had been in Chongqing or Xi’an • The largest time gap along a political border is the 3.5 hour gap along the border of China (UTC +8) and Afghanistan (UTC+4:30)

15. Time Zones 6 • There are many places where three time zones meet, like the tri-country border of Finland, Norway and Russia • Stations in Antarctica generally keep the time of their supply base

16. International Date Line 1 • First discovered when Magellan's expedition sailed around the world in 1522 • Made famous in “Round the World in 80 Days” by Jules Verne • The Philippines used to be a Spanish colony, and had its most important communication with Mexico, and was therefore on the east side of the date line • By 1844, trade interests had changed to China, so it switched to the west side

17. International Date Line 2 • There are time all together 40 time zones ranging from UTC –12 to UTC +14 • -12 are uninhabited islands belonging to the US • In 1995 Kiribati (Gilbert Islands) changed from −11 and −10 to +13 and +14

18. Daylight Saving Time 1 • Introduced in Britain in 1916 • Spring forward, fall back • More efficient use of early morning sunshine • More time for outdoor leisure activities in the afternoon and evening • Saves energy, lowers crime, improves traffic safety • Not used in the winter, because then Sun rises late anyway

19. Daylight Saving Time 2 • In Europe DST is from the last Sunday in March to the last Sunday in October • In the US DST is from the second Sunday in March to the first Sunday in November • Countries in the southern hemisphere have DST in their summer • Britain used year-round DST from 1968 to 1971 but abandoned it because of its unpopularity in northern regions

20. Daylight Saving Time 3 • Used in China from 1986 to 1991