When does a day start

1
When does a day start?
  set Universal Time using the Greenwich
Meridian (Britannia rules the waves...)
      since "noon" drifts around the Earth, need
International Date Line to make the day change
                (avoid political boundaries for
convenience) the Earth moves around the Sun at
varying speeds, so let's use the "mean solar day"
2
Timekeeping
Fundamentally Based on the Sun Civil or
solar time    Noon the Sun is "overhead"
(actually, on the meridian)      but this is
different at every longitude (that was OK until
railroads)      so let's have "time zones"
(the Sun hits the meridian at noon only in the
middle of a zone)       use "daylight savings
time" to make it get dark later (Sun hits
meridian at 1pm) (but nothing
astronomical happens this is an arbitrary
convention)        
3
Solar vs Sidereal Time
The Solar day is not the same as the Sidereal day
(sidereal means when a star crosses the
meridian)  A sidereal day is 4 minutes shorter
(due to Earth's orbiting Sun),        so stars
come up 4 minutes sooner every (solar) day
4
Month and Year
the Moon does not go around its cycle in an exact
number of days (synodic period 29.5 days)
     months (from moon) vary between 30,31 days
(use Feb. to fix year)         lunar months will
drift through the year (need extra month every so
often if usually 12)            lunar weeks will
have drifting weekends (religious out of phase
with civil calendar)
the Year is not an exact number of solar days
long (shorter than 365 1/4 by 11m 14s)       we
have leap years to cover the 1/4 day (otherwise
the seasons will slip)        we have more rules
to cover the rest (no leap year every fourth
century, etc.)            the rotation of the
Earth is slowing down, so need leap seconds too
5
History of Calendars

• The Week
• 7 days about the time between major moon phases
• Sunday - Sun, MondayLundi Moon,
  TuesdayMartedi Mars,
• WednesdayMercoldi Mercury, ThursdayGiovedi
  Jupiter,
• FridayVenerdi Venus, Saturday Saturn
• The Month
• Early Roman 12 lunar months 354 days every 3
  years have a 13th month
• Julian Calendar go to months of 30, 31days
  have leap years to fix extra quarter day
• To fix drift of equinox, 46BC had 445 days (year
  of confusion)
• July and August named after Julius Caesar and
  Augustus Caesar
• The Gregorian Calendar
• Equinox had slipped again by 10 days in 16th
  century Pope Gregory XIII fixed it in 1582 by
  dropping 10 days in Oct.
• changed calendar rules (only century years
  divisible by 400 are leap years)
• Protestants didnt follow until later 1752 for
  US UK (cant charge rent for missing days)
• Year starts on Jan. 1 instead of Mar. 25 (so 1751
  had no Jan., Feb., or Mar 1-24) Washingtons
  birthday not really on Washingtons birthday
  (born Feb. 11 but celebrated Feb. 22)
• Russian didnt change until 1917 revolution
  (losing 13 days)

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Summer, Winter, and the Tropics
The Sun will be overhead on the Tropic of Cancer
on the summer solstice (Northern Hemisphere), and
overhead at the Equator on the equinoxes. From a
pole-up point of view, it shines down from the
North during the summer, and up from the South in
winter. The Poles are only illuminated one at a
time.
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The Reasons for Seasons
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The Midnight Sun
Above the Arctic Circle, the Sun never sets
during summer. It travels a tilted circle around
the sky (at the Pole the circle is flat).
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Why Winter is Colder, even though the Sun is up
Days are shorter, but the Sun is also less
effective at heating the ground. That is because
the sunlight is spread over a greater area, so a
given area gets a smaller fraction of energy.
10
Just kidding.
11
Different Points of View
You can choose to make the Ecliptic horizontal
or you can make the Equator horizontal. You
could also replace the Sun with the Earth in this
picture (going geocentric) and then it would be
clear why the Sun shines down or up on either
pole during the solstices.
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Astro Quiz
Suppose the Earths rotation axis was
perpendicular to its orbital plane (so the
celestial equator was on the ecliptic). Which
statement below would be FALSE?

• There would not be any real seasons.
• All stars would be circumpolar.
• The length of the day would not vary by month or
  latitude.

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What the seasons are NOT due to
NOT the ellipticity of the Earths orbit Our
ellipticity is very small, and we are closest to
the Sun in January. You can most easily see this
by remembering that at the same time it is Winter
here, it is Summer in the Southern Hemisphere.
NOT the fact that one pole is closer to the Sun
because the Earths axis is tilted The size of
the Earth is miniscule compared with its distance
to the Sun, so it doesnt matter how things are
oriented, they are all about the same distance
from the Sun.
Note Summer is not warmest at the solstice
(June) because it takes a while for the seasonal
effects to really kick in, so we tend to be
hottest a couple of months later (and coldest a
couple of months after the winter solstice).
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Precession of the Earths Pole
The changing tilt of the Earths pole slowly
changes at what point in the orbit corresponds to
a given season. Thirteen thousand years from now,
winter will occur 6 months later. The calendar
has to be adjusted to make winter stay in the.
winter months. This has also caused the Sun
signs to drift off from their original dates.