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Lecture 6

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

6 pages86 viewsWinter 2013

Course Code
Patrick Mc Graw

of 6
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
8.3 x 60 = 498 seconds.
Light from the moon to the earth:
498 divided by 400 = 1.2 seconds.
Recap: Ecliptic and seasons:
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
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
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
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.
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.
Celestial pole makes a circle over approximately 26,000 years.
Precession of the equinoxes:
Precession makes the poles and equator wobble relative to the
This means that the equinoxes (the places where the ecliptic and
celestial equator meet) also move relative to the stars.
Sidereal Month:
The time taken for the moon to make a full orbit around Earth
(relative to the fixed stars) 1 sidereal month = 27.3 days.
Synodic Month:
The time between two successive new moons. (longer than sidereal
month due to Earth’s orbital motion around the sun) 1 synodic
month = 29.5 days
Moon Phases: Summary:
o dark, close to sun, not visible at night.
Waxing Crescent: l
o less than half lit, east of the sun, visible in the wet after
First quarter:
o western (right) half lit, visible near the meridian at sunset,
sets around midnight.
Waxing Gibbous:
o more than half lit, visible in the east after sunset, sets after
o opposite side of the sky from the sun, rises near sunset an
visible at night.
Waning Gibbous:
o more than half lit, rises between sunset at midnight.
Third quarter:
o Eastern (left) half lit, rises around midnight.
Waning Crescent:

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