Download Seasons:

Astronomy – Motions in the Sky – Notes
Kowinsky
Seasons:
Seasons ARE NOT caused by the changing distance between
the sun and earth. In reality, during northern hemisphere
winter, earth is slightly closer to the sun than normal.
Seasons are caused by the TILT of the Earth. The earth is
tilted 23.5 degrees on it’s axis of rotation. Because of this,
different hemispheres of the earth receive different amounts
of direct sunlight throughout the year.
Refer to the following diagram:
Seasons are reversed in the
southern hemisphere. During
summer in the North, it is winter
in the South. During winter in
the North, it is summer in the
South.
Since we are on Earth, we do not perceive the Earth moving. Instead, it appears the sun
moves. It appears to travel up and down along the celestial sphere on the ecliptic.
During summer, we get more direct sunlight, so the sun appears to be higher in the sky.
During winder, we get less direct sunlight, so the sun appears to be lower in the sky.
This perceived motion of the sun on the celestial sphere is how we get our seasons.
Summer Solstice: June 21st - 22 When the sun reaches the highest northerly point on the
celestial sphere. This marks the first day of summer, and the longest day of the year.
Fall (Autumnal) Equinox: Sept 22-23rd. When the sun crosses the celestial equator
heading south. This marks the first day of fall, and day and night are equal.
Winter Solstice: December 21 – 22nd. When the sun reaches the lowest point on the
celestial sphere. This marks the first day of winter, and the shortest day of the year.
Spring (Vernal) Equinox: March 20-21st. When the sun crosses the celestial equator
heading north. This marks the first day of spring, and day and night are equal.
In the southern hemisphere, since seasons are reversed, these dates are reversed:
Example: The summer solstice, June 20-21 is the southern hemispheres winter solstice.
Astronomy – Motions in the Sky – Notes
Kowinsky
The Moon:
Tidal Locking:
Although the moon rotates AND revolves, people on Earth only see 1 side of the moon.
We never see the “far” side of the moon.
This is due to a phenomenon called phase locking, or tidal locking.
- The moon is Tidally locked with earth.
Whis means that 1 rotation of the moon is equal to 1 revolution of the moon.
The Moon is by no means the only object which is phase-locked; a large number of the
satellites of other planets also share this property.
Name
O. Per.
(days)
Incl.
Tilt Eccen.
R. Period M.O.V.
(days) (km/s)
Earth
365.256 0.000 23.45 0.0167 0.99727
Moon
27.32166 5.1454 1.5424 0.05
Mars
686.98
Phobos
0.31910 1.0
?
0.01
0.31910
2.14
Deimos
1.26244 0.9-2.7 ?
0.00
1.26244
1.35
29.79
27.32166 1.02
1.850 25.19 0.0934 1.025957 24.13
Jupiter 4332.71 1.308 3.13
0.0483 0.41354
Amalthea 0.498179 0.40
?
0.003 0.498179 26.47
Thebe
0.674536 1.0659 ?
0.0183 0.674536 23.92
Io
1.769138 0.040 ?
0.004 1.769138 17.33
Europa
3.551181 0.470 ?
0.009 3.551181 13.74
Ganymede 7.154553 0.195 ?
0.002 7.154553 10.88
Callisto
16.68902 0.281 ?
Saturn
10759.50 2.488 25.33 0.0560 0.44401
Epimetheus 0.6942
0.6945
Janus
13.06
0.007 16.68902 8.21
9.66
0.34
?
0.009 0.6942
15.86
0.14
?
0.007 0.6945
15.86
0.942422 1.53
?
0.0202 0.942422 14.32
Enceladus 1.370218 0.02
?
0.0045 1.370218 12.63
Tethys
1.887802 1.09
?
0.0000 1.887802 11.35
Dione
2.736915 0.02
?
0.0022 2.736915 10.03
Mimas
Astronomy – Motions in the Sky – Notes
Kowinsky
Rhea
4.517500 0.35
?
0.0010 4.517500 8.48
Titan
15.94542 0.33
?
0.0292 15.94542 5.57
Hyperion 21.27661 0.43 ?
Iapetus 79.33018 14.72 ?
Pluto
Charon
90800
0.1042 chaotic
0.0283 79.33018 3.26
17.148 122.52 0.2482 -6.3872
6.38725 98.80 ?
5.06
0.00
6.38725
4.74
0.22
The basic reason for this phenomenon is the tidal force. The tide of the ocean is well
known; less well-known but equally real is the tidal distortion of the entire planet --- the
continents (and everything underneath) are deformed daily by the tidal force (of the Sun
and the Moon), roughly by the same height as the tide of the ocean. A moon close to a
planet is (relatively speaking) subject to a much greater tidal force than the Earth.
If a moon has two, diametrically opposed, permanent tidal bulges, then it's dynamically
most stable if one is always pointed towards its parent planet.
Perigee and Apogee:
No orbits are perfectly circular. Instead, they are elliptical. This means the the moons
orbit makes a slight ellipse. When it’s closest to earth, the moon is said to be at Perigee
(356,000 km). When it’s farthest from earth, the moon is said to be at Apogee (407,000
km).
Astronomy – Motions in the Sky – Notes
Kowinsky
Phases of the Moon:
The Sun always shines on ½ of the moon. However, depending on the angle at which we
view the moon, we will see different parts of the moon lit up at different times. These are
the phases of the moon. There are 8 official phases of the moon.
One may notice the Full moon is only visible on the night side of Earth, and the New
moon is only visible on the day side of Earth. The first and 3rd quarter are visible during
½ the day, and ½ the night.
Astronomy – Motions in the Sky – Notes
Kowinsky
Eclipses:
Since the Earth has a 23.5 degree tilt, the Moon also orbits at a 23.5 degree angle.
Because of this, the moon is usually above or below the Earth during a full and new
moon. However, twice in the earths orbit, the moon will be in the same plane as the sun
and earth. When this occurs and it’s a full or new moon, the moon will either cast a
shadow on earth (Solar eclipse), or the moon will enter the earth’s shadow (Lunar
eclipse).
Lunar Eclipse:
A lunar eclipse occurs when the earth casts a shadow on the full moon.
Umbra: The central dark shadow of the earth.
Penumbra: The secondary, lighter, shadow of the earth.
 Total Eclipse - When the entire moon enters the
Earth's umbra (the darkest part of its shadow), this is
called a total eclipse.
 Partial Eclipse - When only part of the moon enters
the Earth's umbra, this is called a partial eclipse.
Lunar eclipses occur twice a year, however, only certain parts of the earth can view the
eclipse, the part that is facing the full moon.
Astronomy – Motions in the Sky – Notes
Kowinsky
Solar eclipses:
A Solar eclipse occurs when the moon casts a shadow on the Earth.
This will only occur during a new moon, twice a year.
Umbra: The central dark
shadow of the moon.
Penumbra: The secondary,
lighter, shadow of the
moon.
 Total Eclipse – When the umbra of the moons shadow passes over
the earth. Anyone standing in this umbra shadow on earth will see
the moon completely cover the sun. When it is completely covered,
one will see the faint outer atmosphere of the sun, called the Corona.
 Partial Eclipse – When the penumbra of the moons shadow passes over
the earth. Anyone standing in this penumbra, will not be at the correct angle
to see a total eclipse. Instead, they will see the moon partially cover the sun.
 Annular Eclipse – Occasionally, the moon will pass directly in front of
the sun, so that people on earth may be in the umbra of the moons shadow.
However, they will not see a total eclipse. Instead, they will see a smaller
moon directly in front of the sun with a larger sun peaking from the sides of
the moon.
This occurs when the moon is at Apogee. At apogee, the moon is far away,
and thus appears smaller than normal. It is not large enough to completely cover
the sun. Therefore, the background sun will still be seen as a ring. This is often
called a “ring of fire”
Although there are two solar and lunar eclipses per year somewhere on earth. Lunar
eclipses are much more common to see. This is because in order to see a total solar
eclipse, one must be in the TINY umbra shadow of the moon. Whereas, to see a lunar
eclipse, a person must only be on the night side of the earth (roughly half the earth).
Astronomy – Motions in the Sky – Notes
Kowinsky
Retrograde Motion of Planets:
Observations of Mars spaced a
few weeks apart reveal motion
relative to constellations in the
zodiac. Mars usually moves from
the West to the East compared to
these "fixed stars," but
occasionally changes direction,
moving from East to West for a
few weeks before returning to its
more normal motion:
Since antiquity, astronomers have observed all of the visible planets (Mercury, Venus,
Mars, Jupiter, and Saturn) undertake similar motions. This motion has been problematic
for incorrectly explained for 1000’s of years.
Retrograde Motion: The planets generally move from west to east in the sky.
Occasionally, one of the planets seems to slow down, stop, and loop backwards, moving
from east to west for a short time. After a short time, all the reverse happens and all is
back to normal.
Cause of retrograde motion:
The planet did NOT stop or move backwards in its orbit; for a while, the Earth was
moving faster than the planet, overtaking the planet for a short time, making it seem to
lag. This is because as the planet orbits the sun, the Earth also orbits the sun.
Occasionally, we will overtake the planet in its orbit, and the planet will appear to move
backwards in the night sky.
The following diagram
shows the above
explanation: