Transcript Week 4

Precession of the Equinox
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The Earth’s axis changes direction in a
clockwise direction with a cycle of about
26,000 years
Precession of the Equinox 2
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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
Precession of the Equinox 3
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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”
Precession of the Equinox 4
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The equinox moves clockwise along the
ecliptic
The NCP traces out a circle in the sky
Precession of the Equinox 5
Precession of the Equinox 6
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Most of the time there will not be a
good pole star
We are lucky to have Polaris in our age
Solar and Sidereal Day
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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
Solar and Sidereal Day 2
Tropical and Sidereal Year
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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
Time Zones
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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
Time Zones 2
Time Zones 3
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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
Time Zones 4
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Before 1949, China had five time zones
Time Zones 5
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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)
Time Zones 6
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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
International Date Line 1
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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
International Date Line 2
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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
Daylight Saving Time 1
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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
Daylight Saving Time 2
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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
Daylight Saving Time 3
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Used in China from 1986 to 1991