lecture 2: motions of the earth and...
TRANSCRIPT
Lecture 2: Motions of the Earth and Moon
Astronomy 111
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Motions of the Earth
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Stars, Sun, Moon and planets move in counterclockwise circles around north (south) celestial pole.
Objects generally move east to west when above the horizon (“rising” in east, “setting” in west).
What causes these circular motions?
Observation:
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Motions of the Earth
• The Earth rotates on its axis
• This motion causes: • Sunrise and
sunset • Motion of stars
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Stars rise and set
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Observation:
• One star in the sky never moves – the “North Star” or Polaris
• Why is Polaris fixed in position?
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Earth’s axis is fixed
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Where was this
picture taken?
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Celestial navigation made simple
At Earth’s North Pole: Polaris is directly overhead
At Earth’s Equator: Polaris is due north, on the horizon
In Earth’s Northern hemisphere: Polaris is due north - height above the horizon (in degrees) is equal to your latitude (in degrees)
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Earth’s rotation causes apparent motion
• Remember: the stars and Sun DO NOT orbit the Earth once per day
• Rather, the Earth rotates once per day – Produces apparent motions of stars, Sun,
planets
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The celestial sphere
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Thinking about the stars we see in the night sky
The celestial sphere
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The celestial sphere: a large imaginary sphere around Earth
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Special locations on the celestial sphere
North Celestial Pole = point directly above Earth’s North Pole (near the star Polaris)
South Celestial Pole = point directly above Earth’s South Pole (no nearby bright star)
Celestial Equator = circle directly above Earth’s Equator
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Locations on celestial sphere
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Distances between points on the celestial sphere are measured in degrees, arcminutes, and arcseconds
360 degrees in a circle 60 arcminutes in a degree 60 arcseconds in an arcminute
½ degree = angular size of Sun & Moon
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The night sky
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The sky is divided into 88 constellations
• Mesopotamia, circa 3000 BC: oldest know constellations
• Ptolemy, 2nd century AD: 48 constellations in northern sky
• 16th to 18th century AD: unmapped (mostly southern) regions of sky filled in – Telescopium, Microscopium, Reticulum!
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Modern usage still retains names of stars and rankings by brightness within a constellation, although Arabic names are also used for the
brightest stars
Hipparchus
Hipparchus (ca. 140 BC) was perhaps the greatest astronomer of ancient times. He produced a catalogue of 1000 stars and classified them according to
their apparent brightness.
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Constellations are arbitrary
Other cultures, other constellations:
Stars in a constellation usually are not at the same distance from us.
At a different place in our Galaxy, we would see different star patterns.
Emu Southern
Cross
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Observation:
Sun moves west to east relative to stars (about 1 degree per day).
What causes this annual motion?
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Two possible explanations of 1 year cycle:
HYPOTHESIS #1 Sun revolves around
Earth at a slightly slower rate than the
celestial sphere.
WRONG!
HYPOTHESIS #2 Earth revolves around
the Sun, once per year.
RIGHT!
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A model of the
celestial sphere,
showing the
constellation
borders, the
celestial equator,
and the apparent
path of the Sun
against the
background of
constellations.
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The Sun appears to move west to east relative to stars (1 year cycle)
Today the Sun is “in” a particular constellation, next month in a different one, etc.
Sun’s path on the celestial sphere = ecliptic
Constellations through which the ecliptic runs = zodiac
The ecliptic is NOT the same as the celestial equator!
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The Sun is in the direction of the constellation
Virgo in September, as
viewed from the Earth.
That is why people born in September are said to be born
under the astrological
sign of Virgo.
The Zodiac
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Observation:
The weather in the summer is hotter than in the winter (especially in Texas!)
What is the cause of the seasons on Earth?
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Remember that the Earth's axis of rotation is tilted 23.5 degrees
to the plane of its orbit about the Sun.
Earth’s tilt causes seasons
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Earth’s tilt causes seasons
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When the northern hemisphere is tilted towards the
Sun we have summer in the northern hemisphere and
winter in the southern hemisphere.
When the northern hemisphere is tilted away from
the Sun we have winter in in the northern hemisphere
and summer in the southern hemisphere.
This is the cause of the seasons.
Earth’s tilt causes seasons
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Earth’s tilt causes seasons
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The reason it is warmer in summer than in winter
is that the sunlight is more concentrated on the
ground when the Sun is higher in the sky.
Earth’s tilt causes seasons
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The first day of spring occurs about March 20th or March 21st.
(In the Northern hemisphere, this is called the vernal equinox.)
The first day of autumn occurs about September 22nd. (This would be the first day of spring in
The Southern hemisphere, and the autumnal equinox in the Northern hemisphere.)
The key thing is that the equinoxes occur when the declination of the Sun is zero degrees (i.e.
located on the celestial equator).
Seasons
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“Observation”:
The location of Polaris moves (over thousands of years)… the North Star was not always the North Star!
What causes this motion?
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Spinning top takes <1 sec!
to precess...!
Earth takes 26,000 years!!
Precession
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Hipparchus compared the coordinates of some stars with records made by the Babylonians and discovered that the ecliptic longitudes of the stars were increasing with time, about 1 degree per century. (The modern value is about 1 degree in 72 years.) This is called precession (not to
be confused with the word “precision”). The Earth turning on its axis is like a spinning top.
Precession
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Precession
While Polaris is close to the north celestial pole now, it was not always the case. Due to the 26,000 year period of precession, many stars take their turns being the pole star.
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Observation:
• There is a large bright object in the nighttime (and daytime) sky
• How does the Moon move?
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Motions of the Moon
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The Moon appears to move west to east relative to stars (1 month cycle)
Today the Moon is “in” some constellation
In two weeks: something different In four weeks: first one, again.
Observation: Moon moves west to east relative to stars, taking 27.3 days to complete cycle.
What causes this monthly motion?
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Explanations of 1 month cycle:
HYPOTHESIS #1 Moon revolves around Earth at a
significantly slower rate than the
celestial sphere.
WRONG!
HYPOTHESIS #2
Moon revolves around Earth, once
per month.
RIGHT!
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(not to scale)!
The Moon’s orbit
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Phases of the Moon
• The moon’s phases are due to its motion relative to the Earth and Sun
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Observation:
• Both the Sun and the Moon may be eclipsed in their orbits
• We live in a very special place in order for this to happen…
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Solar eclipses
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Solar eclipses
Solar eclipses occur when Moon is between Sun and Earth.
Solar eclipses occur at NEW MOON.
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Total Solar Eclipse:
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Solar Eclipses
Only a small part of the earth observes a Total Solar Eclipse!
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Total Lunar Eclipse
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Lunar eclipse
Lunar eclipses occur when Earth is between Sun and Moon.
Lunar eclipses occur at FULL MOON.
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Solar vs. Lunar eclipses
Total solar eclipse is visible from a narrow path (up to 270 km wide) for a short time (up to 7.5 min).
Total lunar eclipse is visible from half the Earth.
From Texas, next total lunar: Dec 21, 2010; next total solar: Apr 8, 2024
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Observation:
• The level of the ocean at the seashore rises and falls at different times during the day
• What causes tides?
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Tides are caused by the difference between the Moon’s gravitational force on different sides of the Earth
Time between high tides = 12 hours,
25 min Time between moonrises = 24 hours, 50 min
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Tides
The gravitational force between two objects decreases as the distance between them increases.
The Moon’s gravitational pull on an object will be 7% greater on the closer side of the Earth than on the further side.
If the Moon’s pull were constant, then Earth would be undistorted.
After subtracting average pull, Earth is stretched in Moon’s direction.
Result: TWO tidal bulges, on opposite sides of Earth.
Tides
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Why do we notice tides at the seashore?
Rock is stiff: Tidal bulges in rock are only 0.3 m high.
Water is fluid: Tidal bulges in water are 1 meter high.
Water bulges rise above rock bulges.
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The Sun also creates tides on Earth
High tides are highest when Sun, Earth and Moon line up (called ‘spring tide’).
High tides are lowest when Sun, Earth & Moon are at right angles (‘neap tide’).
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Tidal forces are slowing the Earth’s rotation and enlarging the Moon’s orbit
The ocean’s tidal bulges press down on the ocean floor.
Friction robs energy from Earth’s rotation and uses it to heat the ocean.
This process is known as “tidal breaking”. (Think of the tidal bulges as brake pads!)
The length of the day is increasing by 0.002 seconds per century.
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The Moon rotates about its axis as it revolves around the Earth
On Earth, we always see the same side of the Moon (“near side”).
The “far side” is always turned away from us.
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The Man in the Moon
Moon creates tidal bulges on Earth: Earth creates BIGGER bulges on Moon.
The Moon has already undergone tidal braking.
The Moon’s rotation has slowed to the point where its rotation period equals its orbital period.
This is why the Moon always keeps the same side turned to us.
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Goodbye Moon
Friction between the tidal bulges and ocean floor drags the bulges in the direction of the Earth’s rotation.
Bulges lead Moon by about 10 degrees.
The leading bulge steadily tugs the Moon into a larger orbit.
The average Earth-Moon distance is increasing by 4 meters per century.
How do we know the distance is increasing?
Measure the distance to the moon with great accuracy and watch it change!
Lunar Laser Ranging Experiment
(1969, Apollo 11)
Goodbye Moon