Class notes
Astr 114 Fall 21 Notes
- Course
- ASTR 114
- Institution
- State University Of New York - Binghamton
This is a comprehensive and detailed note on fall 2021. *Essential Study Material!!
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ASTR 142Planet- a moderately large object that orbits a star; it shines by reflected light. Planets
may be rocky, icy, or gaseous in composition
Moon (or Satellite)- an object that orbits a planet
Asteriod- a rel. small and rocky object that orbits a star
Comet- a relatively small and icy object that orbits a star
Solar (star) system- a star and all the material that orbits it, including its planets and
moons
Nebula- a interstellar cloud of gas and/or dust
Galxy- a great island of stars in space, all held together by gravity and orbiting a
common center
Universe- a sum total of all matter and energy that is, everything within and between all
galaxies
Light travels at a finite speed (300,000 km/s)
The farther away we look in distance, the further back we look in time.
Light-year-The distance light can travel in 1 yr
At great distances, we see objects as they were when the universe was much younger
What is our place in the universe?
Earth is part of the solar system, which is the Milky Way Galaxy, which is a member of
the Local Group of galaxies in the Local Supercluster
The distances between planets are huge compared to
their sizes—on a scale of 1-to-10-billion, Earth is the
size of a ball point and the Sun is 15 meters away.
– On the same scale, the stars are thousands of
kilometers away.
– It would take more than 3000 years to count the stars
in the Milky Way Galaxy at a rate of one per second,
and they are spread across 100,000 light-years.
– The observable universe is 14 billion light-years in
radius and contains over 100 billion galaxies with a
total number of stars comparable to the number of
grains of sand on all of Earth's beaches.
Stars at different distances all appear to lie on the celestial sphere.
The 88 official constellations cover the celestial sphere.
The Ecliptic is the Sun's apparent path through the celestial sphere.
North celestial poleis directly above Earth's North Pole.
South celestial poleis directly above Earth's South Pole.
Celestial equatoris a projection of Earth's equator onto sky.
The Local Sky
An object's altitude (above horizon) and azimuth (direction along horizon) specify
its location in your local sky.
Meridian: line passing though zenith and connecting N and S points on horizon
-Horizon all points 90 degrees away from zenith
-Zenith: the point directly overheard
Right ascension: like longitude on celestial sphere (measured in hours with respect to
spring equinox)
,Declination: like latitude on celestial sphere (measured in degrees above celestial
equator)
Right Ascension: Vega’s RA of 18h 35.2m out of 24h places it most of the way around
celestial sphere from spring equinox.
Declination: Vega’s dec of +338degree 44’ puts it almost 39 degrees N of celestial
equator (negative dec would be S of equator).
The Sun’s RA and dec change along the ecliptic during the course of a year.
Sun’s declination is negative in fall and winter and positive in spring and summer.
Earth rotates from W to E, so stars appear to circle from east to west
Stars near the North Celestial pole are circumpolar and never set.
We cannot see stars near the S celestial pole.
All other stars (and Sun, Moon, planets) rise in E and set in W.
As the Earth orbits the Sun, the Sun appears to move eastward along the ecliptic.
At midnight, the stars on our meridian are opposite the Sun in the sky.
We can see over 2000 stars and the Milky Way with our naked eyes, and each position
on the sky belongs to one of 88 constellations
We can specify the position of an object in the local sky by its altitude above the
horizon and its direction along the horizon.
Why do stars rise and set? B/c of Earth’s rotation
Why do the constellations we see depend on time of year? The time of year
determines the location of the Sun on the celestial sphere
Seasons depend on how Earth’s axis affects the directness of sunlight
Earth’s axis points in the same direction (to Polaris) all year round, so its orientation
relative to the Sun changes as Earth orbits the Sun
Summers occurs in your hemisphere when sunlight hits it more directly; winter occurs
when the sunlight is less direct.
AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth
Variation of Earth-Sun distance is small- abt 3%; this small variation is overwhelmed by
the effect of axis tilt.
Variation in any season of each hemisphere- Sun distance is even smaller!
Summer (June) solistice
Winter (December) solstice
Spring (March) equinox
Fall (September) equinox
Although the axis seems fixed on human time scales, it actually precesses over about
26,000 years
- Polaris won’t always be the North Star
- Positions of equinoixes shift around orbit, e.g., spring equinox, once in Aries,
is now in Pisces
- Earth’s axis precesses like the axis of a spinning top
What causes the seasons?
- The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth
more directly during the summer and less directly during the winter
- We can specify the position of an object in the local sky by its altitude above
the horizon and its direction along the horizon
, - The summer and winter solstices are when the N. Hemisphere gets its
most and least direct sunlight, respectively. The spring and fall equinoxes
are when both hemispheres get equally direct sunlight
How does the orientation of Earth’s axis change with time?
- The tilt remains about 23.5 degrees (so the season pattern is not affected),
but Earth has a 26,000-year precession cycle that slowly and subtly changes
the orientation of Earth’s cycle
Sidereal day: Earth rotates once on its axis in 23 hrs, 56 mins, and 4.07 secs
Solar day: The Sun makes one circuit around the sky in 24 hrs
A solar day is longer than a sidereal day by 1/360 because Earth moves about 1 degree
in orbit each day.
Sidereal month: Moon orbits earth in 27.3 days. Earth and Moon travel 30 degrees
around Sun during that time (30/36=1/12)
Synodic month: A cycle of lunar phases; takes about 29.5 days, 1/12 than a sidereal
month
Sidereal year: Time for Earth to complete one orbit of Sun
Tropical year: Time for Earth to complete one cycle of seasons. Tropical year is about
20 mins (1/26,000) shorter than a sidereal year b/c of precession
Planetary periods can be measured with respect to stars (sidereal) or to apparent
position of Sun (synodic)
Differences between a planet’s orbital (sidereal) and synodic period depends on how far
planet moves in 1 earth year
Po= Psync (1yr/Psync-1yrs)
The length of a tropical year is about 365.25 days
In order to keep the calendar year synchronized with the szns, we must add one day
every 4 years (Feb 29)
For precise synchronization, years divisible by 100 (e.g., 1900) are not leap years
unless they are divisible by 400 (e.g., 2000)
Lunar Phenomena
First spacecraft to fly past Moon: January 1959
First spacecraft to (crash) land on Moon: September 1959
First pic of far side of Moon: October 1959
The US is (so far) the only country to send people to the Moon
First person on Moon: July 1969
Last person on Moon: December 1972
Lunar phases are a consequence of the Moon’s 27.3-day orbit around Earth
Half of Moon is illuminated by Sun and half is dark
We see a changing combo of the bright and dark faces as Moon orbits
Moon takes about 29.5 days to go through the whole cycle of phases (synodic month)
Phases are due to different amounts of sunlit portion being visible from Earth
Time to make full 360 degrees rotation around Earth, sidereal month, is about 2 days
shorter
Waxing- moon visible in afternoon/evening
Gets “fuller” and rises later each day
Waning- moon visible in late night/morning
Gets “less full” and sets later each day
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