A Quick Tour of the Universe (and this course) Part 2
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Transcript of A Quick Tour of the Universe (and this course) Part 2
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A Quick Tour of the Universe
(and this course)Part 2
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Charting the Heavens: Foundations of Astronomy
Learning Goals• Describe the Celestial Sphere and how astronomers use
angular measurement to locate objects in the night sky.
• Account for the apparent motions of the Sun and the stars in terms of the actual motion of the Earth. Explain why our planet has seasons.
• Understand the changing appearance of the Moon and how the relative motions of the Earth, the Sun, and the Moon lead to eclipses.
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Clicker Question:
If Earth rotated twice as fast as it currently does, but its motion around the sun stayed the same, then which of the following is true:A: the night would be twice as long
B: the night would be half as long
C: the year would be half as long
D: the year would be twice as long
E: the length of a day would be unchanged
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Clicker Question:
If Earth rotated twice as fast as it currently does, but its motion around the sun stayed the same, then which of the following is true:
B: the night would be half as long
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Clicker Question:
Distances to nearby stars are measured by:A: bouncing radar signals off them
B: using laser beams
C: using geometry and parallax measurements
D: measuring how long it took a spacecraft to get there and back and assuming a constant velocity.
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Clicker Question:
Distances to nearby stars are measured by:
C: using geometry and parallax measurements
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Clicker Question:
How many stars are there in the observable universe?A: 1012
B: 1022
C: 1032
D: 1042
E: infinite
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Clicker Question:
How many stars are there in the observable universe?
B: 1022
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The Sky at Night
What do we see?
The MoonPlanetsPerhaps a meteor shower, comet, or other rare eventStars - about 3000 visiblePatterns of stars - constellations 88 of them Useful for finding our way around the sky, navigating the oceansSatellites, airplanes, clouds, lightning, light pollution ...
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The Celestial Sphere
Features:- Does not rotate with Earth- Poles, Equator- Coordinate System
An ancient concept, as if all objects at same distance.
But to find things on sky, don't need to know their distance, so still useful today.
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The "Solar Day" and the "Sidereal Day"
Solar Day
How long it takes for the Sun to return to the same position in the sky (24 hours).
Sidereal Day
How long it takes for the Earth to rotate 360o on its axis.
These are not the same!
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One solar day later, the Earth has rotated slightly more than 360o .A solar day is longer than a sidereal day by 3.9 minutes(24 hours vs. 23 hours 56 minutes 4.091 seconds).
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Clicker Question:
Why is it warmer in Albuquerque in the summer than winter?A: The northern hemisphere is tilted towards the sun in summer.
B: The Earth is closer to the sun in summer.
C: The greenhouse effect increases in summer.
D: The sun increases its intrinsic luminosity in the summer.
E. All of the above.
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Clicker Question:
Why is it warmer in Albuquerque in the summer than winter?A: The northern hemisphere is tilted towards the sun in summer.
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The Earth's rotation axis is tilted with respect to its orbit around the Sun => seasons.
Summer Winter
Scorpius Orion
Tilt is 23.5o
DayNight Day NightSun high in northern sky
Sun low in northern sky
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Winter
In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler
Summer
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Summer Winter
Scorpius OrionDayNight Day Night
The Year
The Earth revolves around the Sun in 365.256 days (“sidereal year”). But the year we use is 365.242 days (“tropical year”). Why?
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Precession
The Earth has a bulge. The Moon "pulls down" on the side of the bulge closest to it, causing the Earth to wobble on its axis (how do we know this?)
Earth
Moon
Spin axis
**Vega Polaris
Precession Period 26,000 years!
Precessionanimation
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The Motion of the Moon
DEMO - Precession
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Winter: July or January?
Winter: January
Scorpius OrionDayNight Day Night
Scorpius OrionNightDayNight Day
Summer: January or July?
Summer: July
Now
13,000 years from now
We choose to keep July a summer month, but then in 13,000 years, summer occurs on other side of orbit!
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The Motion of the Moon
The Moon has a cycle of "phases", which lasts about 29 days.
Half of the Moon's surface is lit by the Sun.
During this cycle, we see different fractions of the sunlit side.
Which way is the Sun here?
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See Tutorial on book’s websitefor animation.
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The Motion of the Moon
DEMO - Phases of the Moon
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Cycle of phases slightly longer than time it takes Moon to do a complete orbit around Earth.
Cycle of phases or "synodic month"
Orbit time or "sidereal month"
29.5 days 27.3 days
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Eclipses
Lunar Eclipse
When the Earth passes directly between the Sun and the Moon.Sun Earth Moon
Solar Eclipse
When the Moon passes directly between the Sun and the Earth.
Sun EarthMoon
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Solar Eclipses
Total
Diamond ring effect - just before or after total
Partial
Annular - why do these occur?
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Lunar Eclipse
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Why don't we get eclipses every month?
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Moon's orbit tilted compared to Earth-Sun orbital plane:
Sun EarthMoon
Moon's orbit slightly elliptical:
Earth
Moon
Side view
Top view, exaggerated ellipse
Distance varies by ~12%
5.2o
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Types of Solar Eclipses Explained
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Certain seasons are more likely to have eclipses. Solar “eclipse season” lasts about 38 days. Likely to get at least a partial eclipse somewhere. Animation
It's worse than this! The plane of the Moon's orbit precesses, so that the eclipse season occurs about 19 days earlier each year.
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Recent and upcoming total and annular solar eclipses
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Eratosthenes Determines the Size of the Earth in about 200 B.C.
SyeneAlexandria
Sun's rays
7.2o S
N
Earth
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He knows the distance between the two cities is 5000 "stadia".
From geometry then,
7.2o 360o Earth's circumference
5000 stadia =
=> circumference is 250,000 stadia, or 40,000 km.
So radius is:
40,000 km2 pi
= 6366 km
(very close to modern value, 6378 km!)