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5. Celestial Motion
The Earth is Spherical
because of this The Sun, Moon, planets, and stars are at different angles
depending on viewer’s latitude. Equatorial latitudes are warmer than polar latitudes (due to the angle of
the Sun).
The Earth spins (rotates) on its axis because of this the Sun, Moon, planets, and stars daily rise in
the east and set in the west. The spinning Earth gives direction to winds and storms (Coriolis Effect),
and makes the Foucault Pendulum turn. The gooey Earth is fatter at the equator.
Diurnal Motion
The Apparent Motion of the Celestial Sphere due to the Earth’s Rotation
Sidereal Day versus Solar Day
Sidereal Day is the time for the Earth to spin once, relative to the
stars. This takes 23 hours 56 minutes. Every sidereal day, the stars are in the same position as they
were the day before. Solar Day is the time for the Sun to move from noon (highest position) to noon,
an average of 24 hours. Longer than the rotational period, because the Earth moves in orbit one degree
each day. Varies depending on the speed of Earth in orbit. Every solar day, the Sun is in the same
position as it was the day before (or kind of close).
The Earth Revolves (orbits) Around the Sun
because of this the stars seen overhead at midnight shift
during the year. The Earth moves 2 AU from one side of its orbit to the other, allowing us to use
parallax to measure the distance to the nearby stars.
The Earth’s Spin Axis is Tipped 23.4º
because of this The Earth’s Spin Axis is Tipped 23.4º
Because of this the angle of the Sun increases and decreases 47º during the year, so the Sun is higher
or lower in the sky, and days are longer and shorter, causing seasons.
(Earth’s axial tilt, combined with its motion around the Sun, produces the cyclic weather patterns
called seasons. Distance from the Sun does NOT cause the seasons!!)
(This axial tilt also prevents the arctic and Antarctic from receiving any sunlight for periods.)
The Moon orbits the Earth
because of this we see lunar phases. The Moon rises (and sets) 49
minutes later each day. Tides increase and decrease depending on Sun and Moon positions. Eclipses
occur when Sun, Earth, and Moon exactly align.
The Moon’s orbit is tipped to the ecliptic
because of this we don’t get eclipses every month. An
eclipse can occur only when the Sun crosses the Moon’s orbital plane—called an eclipse season. We
have an eclipse season about twice a year.
Lunar Eclipse: The Moon passes through the Earth’s shadow, taking several hours, seen by everyone
on Earth’s dark side. This occurs only during a full moon.
Solar Eclipse: Moon’s shadow passes across the Earth’s surface, seen for only a few minutes by people
living in the shadow’s path. This occurs only during a new moon.
The Moon’s period of rotation matches the period of revolution
because of this we see only one
side of the Moon from the Earth
The Planets all orbit in the same direction (counterclockwise, as seen from the north), and in the
same plane
because of this the planets are seen in a band of sky, the zodiac. The planets move
from west to east, relative to the stars, with short periods of retrograde motion.
Retrograde Motion of Mars
Caused by Earth passing Mars on the inside orbit
Mercury and Venus orbit closer to the Sun than the Earth, while the other planets orbit further out
because of this Mercury and Venus are only seen close to the Sun (early evening or early morning),
and they show phases (like the Moon). The other planets can be seen anywhere along the zodiac, and
are always mostly full.
The Earth’s axis precesses, taking 26,000 years
because of this star positions slowly rotate in a 23º
circle (over 26,000 years). Direction of north axis moves so that Vega will be the north star in 11,000
years. One seasonal year is NOT the time it takes to go around the Sun. Tropical (seasonal) year is 20
minutes shorter than the sidereal (revolution) year. Current “winter stars” will NOT be the “winter stars”
in a few thousand years.
The stars are very far away
because of this even though the stars are extremely large, they appear
as only pinpoints (even in the largest telescopes). We see very little proper motion of the stars from
year to year.
Stars move around the galaxy
This illustration shows how Bernard’s Star has been moving for the
past 20 years. Most stars are too far away for their motion to be noticeable.
Questions to be Answered:
•
How does the Earth rotate (spin), and what phenomena does this produce?
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How does the Earth revolve (orbit) and what phenomena does this produce?
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What’s the difference between the solar day and the sidereal day?
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How do the planets move in the night sky, and why?
•
What effects are due to Earth’s tipped rotational axis?
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Why do we have seasons?
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What are the phases of the Moon?
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What are the two kinds of eclipses, how do they occur, how often do they occur, and how do
they look from Earth?
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How does the Earth precess, and what effect does it have?
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How do the stars move? Is this noticeable?