- Terrestrial
- Mercury
- Venus
- Earth
- Mars
- Jovian
- Jupiter - largest
- Saturn
- Uranus
- Neptune
- Other
- Pluto
Local neighborhood of stars
Nearest star = Proxima Centauri (a member of the Alpha Centauri complex, a triple-star system) Distance 271,400 times the distance from the Earth to the Sun. About 270,000 A.U. or 4.3 light years (see p. 278-279)
Milky Way Galaxy
Each galaxy contains about a hundred billion stars.Local Group
Nearest neighboring large galaxy = Andromeda Galaxy (M31). About 2 million light years away. (see p. 54 and p. 417-8). About 20 galaxies are in this area. A group of galaxies held together by their mutual gravitational attraction.
Universe
Everything. The totality of all space, time, matter, and energy.The Universe is expanding.
Doppler Effect
Red shift
Interpreted as Big Bang. Theory. The event that cosmologists consider to be the beginning of the Universe. All matter and radiation came into being. Began in one place. Explosion. Matter still moving outward.
When?
Conventional view = 13 - 15 billion years ago. (see p. 459)
New Hubble Space Telescope Data = 8 billion years ago.
Cycles?
Origin of Solar System
4.6 billion years agoThe Scale of Things
Light year = the distance light can travel in a year.
Speed of light = 299,792.458 km/sec (or about 300,000 km/sec)
= 186,282 mi/sec
A light year = about 10 trillion km or around 6 trillion miles.
See exercise sheet "A Sense of Scale"
Solar system distances
| Earth | 12,765 km | 2160 mi |
|---|---|---|
| Moon | 3,476 km | 2160 mi |
Compare the size of the Earth and Moon.
The distance between the Earth and Moon is 384,401 km (238, 910 mi)
How many Earth diameters is the Moon from the Earth?
The diameter of the Sun is 1,390,ooo km (866,000 mi)
The distance to the Sun is defined as 1 Astronomical Unit (A. U.)
1 A.U. = 149,598,000 km or 92,956,000 mi (commonly referred to as 93 million miles)
This can be expressed in scientific notation as 9.3 x 10 to the 7 miles
Example of solar system distances and travel times: Magellan spacecraft to Venus - launched April 1989. Required 15 months of flight time, arriving in August 1990.
Voyager I and II spacecraft were launched 8/20/77 and 9/5/77.
To Jupiter 3/5/79 amd 7/9/79 (took about 1 and a half years)
To Saturn 8/25/81 and 11/12/81 (3-4 years)
To Uranus ---- and 1/24/86 (9 years)
To Neptune ---- and 8/25/89 (12 years)
Note that Voyage I headed out of the solar system and did not fly by Uranus and Neptune. Voyager II is also now headed out of the solar system.
Constellations
(see p. 6-8)Take the constellation quiz from Mt. Wilson Observatory.
On a clear night, about 3000 stars visible.
Eye connects brighter stars into patterns called constellations.
Named by ancient astronomers for mythological beings, animals, etc.
Orion - easy to spot in Winter
Big Dipper in North. Pointer stars point to Polaris, the North Star or Pole Star. (Not very bright.)
Used in Navigation.
Signs of the Zodiac are constellations. Your "sign" is when the Sun is "in" that particular constellation. Therefore, it would be "up" in the daytime, and is not visible at all, at that time of year. (It would be visible 6 months later on opposite side of the sky from the sun.)
Constellations are used as "reference points" in the sky. They have no mystical power.
There are 88 constellations in all - some are not visible from the Northern Hemisphere.
We see different constellations at different times of the year because the Earth moves around the sun.
Basic terms to know:
ROTATION
REVOLUTION
Rotation is spinning on an axis.
The Earth rotates once a day.
Revolution is orbiting around another body.
The Earth revolves around the Sun.
Celestial Sphere
The celestial sphere is an imaginary sphers of the heavens on which the celestial objects are considered to be attached.
Extend Earth's axis of rotation outward -
North Celestial Pole
South Celestial Pole
The celestial sphere appears to turn about on the axis passing through the celestial poles. This apparent rotation is due to the rotation of the Earth.
The North Celestial Pole is about 1 degree away from the star Polaris. (There is no similar star at the South Celestial Pole.)
The celestial equator lies midway between the N and S celestial poles.
All of this is just to describe what we SEE, not what is actually out in space.
_______________________________
We can use the stars to determine our latitude in the Northern Hemisphere.
At the N Pole, Polaris is straight up (at the ZENITH). 90 degrees up.
At the equator, Polaris is on the horizon. 0 degrees up.
For every degree of latitude that you move toward the N pole, Polaris shifts 1 degree higher above the horizon.
Therefore, the altitude (in degrees) of Polaris above the horizon = your latitude.
Polaris always seems to stay in one point in the night sky (see Fig. 1.8 on p. 9). The other stars seem to rotate around it. This is a result of the Earth's rotation.
Some stars stay above the horizon and never set. These are the circumpolar stars.
Other stars (most stars) RISE and SET. They are called noncircumpolar.
Some basic terms:
ALTITUDE = Angular distance above the horizon.
Measured with a simple sextant.
AZIMUTH = Angular distance around the horizon from the North.
Measured with a compass, in a clockwise direction.
Due N = 0 degrees, due E = 90 degrees, due S = 180 degrees, due W = 270 degrees.
We will be measuring the Altitude and Azimuth of some celestial bodies in the sky for assignments, so it is important to understand these terms.
More basic terms:
ZENITH = The straight up point.
MERIDIAN = An arc passing overhead from due N to due S.
CELESTIAL HORIZON = 90 degrees from the zenith. Would be your horizon at sea, but rarely seen on land unless it is extremely flat.
___________________
NOON is defined as the time at which the Sun is on the meridian.
Can you figure out what a.m. and p.m. mean?
ante meridiem
post meridiem
Daily motions of the Earth
If we point a telescope at a star on the meridian, where will it be 24 hours later?If a day is 24 hours long, it should be in the same spot, BUT IT IS NOT!
How long does it take for the star to return to the same spot? 23 hr 56 min 4 sec
This is the time for the Earth to rotate once on its axis with respect to the stars.
It is called the SIDEREAL DAY.
It is about 4 minutes shorter than a 24 hour day.
Why do clocks keep 24 hour time?
From noon to noon is 24 hours. This is the SOLAR DAY.
BUT THE SOLAR DAY IS NOT EQUAL TO THE SIDEREAL DAY.
WHY?
As the Earth turns on its axis, it also revolves part of the way around the sun, travelling about 3.2 million km or 2 million miles on its year-long journey around the sun EVERY DAY.
This is an angle of 0.986 degrees per day - almost 1 degree. (360 degrees divided by 365 days).
The Sun seems to shift against the background of stars as the days go past. The sun will not be in the same place on the celestial sphere after 1 rotation.
THERE IS A 4 MINUTE DIFFERENCE.
The stars are AHEAD by 4 minutes each day.
(After 2 days = 8 min, after 3 days = 12 min, ..., after 365 days = 24 hours).
Hence, after 1 year (365 days), the stars are back in the same place again.
As a result of this, we see different constellations at different times of the year. The stars RISE and SET 4 minutes earlier each day!
Seasonal Changes
Note that the Earth's axis is tilted at 23 and a half degrees to the orbital plane around the Sun.The Earth orbits the sun in 365.24220 days.
This is the TROPICAL YEAR.
At this point, the Sun returns to its starting position on the celestial sphere. (Note that the .24220 days is equal to 5 hours, 48 minutes and 46 seconds. This is a little less than 6 hours. This is the reason that we have a LEAP YEAR every 4 years.)
The Seasons
More basic terms:
ECLIPTIC = The path the sun follows across the sky during a day.
SUMMER SOLSTICE = The sun is as high or as far north as it gets all year.
First day of Summer (June 21 or 22)
The Sun is directly above the Tropic of Cancer (N-hemisphere)
WINTER SOLSTICE = The Sun is at its lowest, and the farthest south it gets all year.
First day of Winter (Dec 21 or 22)
The Sun is directly above the Tropic of Capricorn (S-hemisphere).
EQUINOX = The Sun is directly above the equator. Rises due E and sets due W.
VERNAL EQUINOX = Spring equinox.
First day of spring (March 21 or 22).
AUTUMNAL EQUINOX = Fall equinox
First day of autumn (Sept 22 or 23).
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Arctic Circle is at 66.5 degrees N latitude.
In summer, 24 hours of daylight. Sun does not rise or set during the day. Rises to a maximum of 23.5 degrees above the horizon at the Summer Solstice. Moves around in a circle above the horizon during the day.
In winter, 24 hours of darkness. Sun stays below horizon.
Sun rises above horizon at North Pole on the Vernal Equinox; sets on Autumnal Equinox.
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This page created by Pamela J. W. Gore
pgore@gpc.edu
January 16, 1996