Precession of the Equinox

1
Precession of the Equinox

• The Earths 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 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

3
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

4
Precession of the Equinox 4

• The equinox moves clockwise along the orbit
• Perihelion moves later in the calendar year
• 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 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

8
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

9
Time Zones

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

10
Time Zones (cont.)
11
Time Zones (cont.)

• 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

12
Time Zones (cont.)

• Before 1949, China had five time zones

13
Time Zones (cont.)

• 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)

14
Time Zones (cont.)

• 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