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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"
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)
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
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
History of Calendars
• The Week
– 7 days : about the time between major moon phases
• Sunday - Sun, Monday[Lundi] – Moon, Tuesday[Martedi] – Mars,
• Wednesday[Mercoldi] – Mercury, Thursday[Giovedi] – Jupiter,
• Friday[Venerdi] – 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 didn’t follow until later; 1752 for US & UK (can’t charge rent for missing days)
– Year starts on Jan. 1 instead of Mar. 25 (so 1751 had no Jan., Feb., or Mar 1-24); Washington’s
birthday not really on Washington’s birthday (born Feb. 11 but celebrated Feb. 22)
– Russian didn’t change until 1917 revolution (losing 13 days)
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.
The Reasons for
Seasons
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).
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.
Just kidding….
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.
Astro Quiz
Suppose the Earth’s rotation axis was perpendicular to its
orbital plane (so the celestial equator was on the
ecliptic). Which statement below would be FALSE?
1) There would not be any real seasons.
2) All stars would be circumpolar.
3) The length of the day would not vary by month or latitude.
What the seasons are NOT due to…
NOT the ellipticity of the Earth’s 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 Earth’s
axis is tilted:
The size of the Earth is miniscule compared with its distance to the
Sun, so it doesn’t 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).
Precession of the Earth’s Pole
The changing tilt of the Earth’s 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.