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Lecture 6

AS101 Lecture Notes - Lecture 6: Equatorial Bulge, Solar Time, Equinox

6 Pages
104 Views
Winter 2013

Department
Astronomy
Course Code
AS101
Professor
Patrick Mc Graw
Lecture
6

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Lecture 6 1/23/2013 12:21:00 PM
Cycles of the Sky, Part II
Homework is due Monday.
Old homework :
The amount of time it takes like to travel from the sun = 8.3
minutes.
8.3 x 60 = 498 seconds.
Light from the moon to the earth:
498 divided by 400 = 1.2 seconds.
Recap: Ecliptic and seasons:
Ecliptic:
o the apparent path of the sun along the celestial sphere during
a year (the sun appears to move eastward along the ecliptic)
Vernal Equinox:
o the point on the celestial sphere where the ecliptic crosses the
celestial equator from south to north, or the time of year
when the sun is at this point
Summer solstice:
o the point where the ecliptic is farthest north, or the time of
year when the sun is at this point
Autumnal equinox:
o the point where the ecliptic crosses the celestial equator from
north to south, or the time of year when the sun is at this
point.
Winter solstice:
o point where the ecliptic is farthest south, or the time of year
when the sun Is at this point.
Summer solstice:
The north pole is facing the sun
Winter solstice:
The north pole is facing away from the sun
Vernal Equinox
When both the north and south poles receive the same amount of
sun
Terminator:
the great circle separating the night side from the day side of any
planet or satellite.
So if you are standing on the earths terminator, what time is it?
o It is either sunrise or sunset.
Reasons for seasons:
What make summer warmer than winter?
o There are two things working together:
Sun is above the horizon for longer each day
Angle of the sun: sun reached closer to the zenith
Sidereal time: Why is right ascension often given in hours:
remember: right ascension is an angle measured east from the
vernal equinox
why is it measured in hours?
An objects right ascension tells us what time it crosses the
meridian.
If one stars right ascension is bigger by an hour than another, then
that star crosses the meridian one hour later.
(This is also true of rising and setting times for stars near the
celestial equator.)
a convenient way for astronomer to describe time: start a 24-hour
clock at 0:00 when the vernal equinox crosses the meridian.
This is called sidereal time. (different from solar time, because the
sun is not always at the vernal equinox.)
So, another way to describe a star’s right ascension: it is the
sidereal time when that star crosses the meridian.
Precession:
Celestial poles and equator wobble slowly in comparison to the
ecliptic and the stars.
Precession is caused by the moon and suns uneven gravitational
pull on the earth’s equatorial bulge.

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Description
Lecture 6 1/23/2013 12:21:00 PM Cycles of the Sky, Part II Homework is due Monday. Old homework :  The amount of time it takes like to travel from the sun = 8.3 minutes.  8.3 x 60 = 498 seconds.  Light from the moon to the earth:  498 divided by 400 = 1.2 seconds. Recap: Ecliptic and seasons:  Ecliptic: o the apparent path of the sun along the celestial sphere during a year (the sun appears to move eastward along the ecliptic)  Vernal Equinox: o the point on the celestial sphere where the ecliptic crosses the celestial equator from south to north, or the time of year when the sun is at this point  Summer solstice: o the point where the ecliptic is farthest north, or the time of year when the sun is at this point  Autumnal equinox: o the point where the ecliptic crosses the celestial equator from north to south, or the time of year when the sun is at this point.  Winter solstice: o point where the ecliptic is farthest south, or the time of year when the sun Is at this point. Summer solstice:  The north pole is facing the sun Winter solstice:  The north pole is facing away from the sun Vernal Equinox  When both the north and south poles receive the same amount of sun Terminator:  the great circle separating the night side from the day side of any planet or satellite.  So if you are standing on the earths terminator, what time is it? o It is either sunrise or sunset. Reasons for seasons:  What make summer warmer than winter? o There are two things working together:  Sun is above the horizon for longer each day  Angle of the sun: sun reached closer to the zenith Sidereal time: Why is right ascension often given in hours:  remember: right ascension is an angle measured east from the vernal equinox  why is it measured in hours?  An objects right ascension tells us what time it crosses the meridian.  If one stars right ascensio
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