Download The Passage of Time Moon, Moonth, Month Sidereal (Star) Time

Lecture 4
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The Passage of Time
• Keeping track of time is one of the most
fundamental tasks of astronomers.
• Various astronomical cycles influence
human activity:
– day: most fundamentally the synodic day
– month: the phases of the Moon
– year: seasonal changes related to the Sun
OWL #1 – Thursday
Put your clicker numbers in OWL
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Moon, Moonth, Month
Sidereal (Star) Time
• The 7 day week may have originated
because it approximates the phases of the
moon (Babylonians and Sumarians)
• Not exact: Moon’s phase cycle is 29.5 days.
• 12 lunar cycles is also close to a full year;
354 days instead of 365 (12 x 29.53059 =
354.36708)
• History of calendars and astronomy shows
social and cultural effects.
• Most stars rise and set like the Sun does; we
can measure time by a star’s position.
• A difference arises because the Earth is
orbiting the Sun….
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Why is the sidereal day shorter?
Work with a partner to create a sketch which
shows why the sidereal day is shorter than
the solar day.
You will need to show the motion of the
Earth/where it is in its orbit for
• (1) when a star crosses the meridian to the
next crossing
• (2) when the sun crosses the meridian to the
next crossing
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Sidereal vs. Synodic (Sun) Time
Earth moves a little bit in its
orbit each day, so the Sun
appears to shift its position
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Tilt affects where the Sun is on the Sky
Star time vs. Sun time
• The Sidereal day is ~4 minutes shorter than
the Solar day.
• There are 365.25 Solar days and 366.25
Sidereal days in a year.
• The Sun shifts with respect to the stars by
about 1 degree each day. Consequently, we
see different constellations in winter than in
summer! Cue Starry Night
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Seasonal
Change in
Sun’s Altitude
Where does the Sun rise different
times of the year?
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The Earth’s axis is tilted
~23.5 degrees relative to its
orbit around the Sun.
The axis remains pointed in
the same direction (toward
Polaris) as Earth orbits.
As a result the Sun shifts
from a declination of 23.5 deg
N to 23.5 deg S over the year
The tilt of the earth and time of year determine direction of sunrise & set.
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PRS: Seasons
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The Sun in January
• Temperature changes are caused by changes
in the total energy of sunlight we receive.
• This can occur because of changes in:
– (1) our distance from the Sun
– (2) the directness of sunlight that we receive
– (3) the number of hours of daylight
• Which of these has a the most impact on
Earth’s seasons?
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The Sun in July
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What part of the sky can you see?
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The North Celestial Pole
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Lost at Sea?
Polaris is just about
as high above the
horizon as your
lattitude.
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