Precession of the Equinox

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Precession of the Equinox

• The Earths axis changes direction in a clockwise
  direction with a cycle of about 26,000 years

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Precession of the Equinox 2

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

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Precession of the Equinox 3

• Discovered by Hipparchus around 150 B.C.E.
• Discovered independently in China around 320 C.E.
  by Yú Xi (??)
• These are the only known discoveries of
  precession
• Dont confuse it with procession

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Precession of the Equinox 4

• The equinox moves clockwise along the ecliptic
• The NCP traces out a circle in the sky

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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 4 min
  shorter than the solar day
• 24 h/360 2460 min/360 4 min
• A year of about 365 solar days has about 366
  sidereal days

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Solar and Sidereal Day 2
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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

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Time Zones

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

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Time Zones 2
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Time Zones 3

• We are at longitude 104, so we would expect to
  be in UTC7, but in fact we are in UTC8
• 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

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Time Zones 4

• Before 1949, China had five time zones

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Time Zones 5

• China could not have enforced a one China
  policy on time zones if the capital had been in
  Chongqing or Xian
• The largest time gap along a political border is
  the 3.5 hour gap along the border of China (UTC
  8) and Afghanistan (UTC430)

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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

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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

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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

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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

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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

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Daylight Saving Time 3

• Used in China from 1986 to 1991