Astronomical by George Hernandez

Basic astronomical measurements plus astronomy units of measure.

Astronomy Units of Measure
Basic Astronomy Measurements

Astronomy Units of Measure

A parsec (pc) stands for "PARallax of one arc SECond". If right triangle EDS has E=Earth, S=Sun, D=Distant object, and angle D is 1 arcsecond, then distance ES is 1 parsec. Roughly 1 pc = (360*60*60/2pi) AU = 206,265 AU = 3.08568e16 m = 30.8568 Pm = 3.2616 ly.
A astronomical unit (AU) is the mean radius of the Earth about the Sun. 1 AU = 149,597,870,691 m ~ 92.96e6 miles ~ 8.317 light minutes.
A light year (ly) is the distance covered by light in 1 year. Light travels at 299,792,458 m/s = 186,282.397 miles/s (or, as usually rounded, 300,000 km/s or 186,000 miles/s), thus 1 ly ~ 9.46e15 m ~ 9.46e12 Km ~ 5.88e12 miles ~ 63,241 AU.

Basic Astronomy Measurements

The Universe
- Age: Roughly 13.7 billion (13.7e9) years.
- Number of Galaxies in the observable universe: Roughly 100 to 200 billion (1 to 2 e11).
- Observable Diameter: 13.7 billion (13.7e9) ly.
The Virgo Supercluster.
- Number of groups and clusters: Roughly 100, including the Local Group.
- Mass: Roughly 10e15 Suns.
The Local Group.
- Number of galaxies: 30, including its 3 largest: The Milky Way galaxy, the M31 (the Andromeda galaxy), and the smaller M33 (the Triangulum galaxy).
- Diameter: Roughly 10 million (10e6) ly.
- Nearest Galaxy: Andromeda Galaxy (aka Messier Object 31, M31, or NGC 224), roughly 2.9e6 ly = 920 Kpc away. It is a spiral galaxy in the direction of the Andromeda constellation.
- Density of Intergalactic Medium (IGM): < 1 atom/ cm ³.
The Milky Way galaxy, aka The Galaxy.
- Number of Stars: From the luminosity of our galaxy, it is estimated that our own Milky Way galaxy has 100 to 300 billion (1 to 3 e11) stars. The Milky Way galaxy has several dwarf galaxies orbiting it.
- Diameter: ~100,000 (1e5) light years.
- Mass: Roughly 1 trillion (1e12) Suns.
- Density of Interstellar Medium (ISM): ~0.5 atoms / cm ³.
Local Bubble:
- Width: 300 light years.
- Distance from center of Galaxy: 8.6 Kpc ~ 26,000 ly ~ 2.5e17 Km
- Location: In the Orion Arm (a minor spiral arm) located between the Perseus Arm and the Sagittarius Arm, two of the four major spiral arms.
- Density: ~0.05 atoms / cm ³.
Local Fluff
- Density: ~0.1 atom / cm ³.
- Contains several star system including our own Sun:
  - Alpha Centauri system. Composed of three stars: The Alpha Centauri binary system has A.C. A and A.C. B at 4.36 ly, while Proxima Centauri ~4.22 ly). The nearest to the sun and 4th brightest besides the Sun. In the constellation Centaurus, visible only from the southern hemisphere.
  - Sirius, ~8.58 ly. The 5th nearest to the Sun and the brightest besides the Sun.
Our Solar System:
- Diameter: Assuming Pluto is the furthest planet, then the diameter is 39.5 AU. --> As of 2006, if we assume that Neptune is the furthest planet, then the diameter is roughly 30 AU.
- Sun to Earth: 1 AU ~ 8.317 light minutes ~ 149,597,870,691 m ~ 149.6e6 Km ~ 92.96e6 miles.
- Major objects:
  - Mercury: 0.4 AU, 0.055 Earth masses.
  - Venus: 0.7 AU, 0.815 Earth masses.
  - Earth: 1.0 AU, 1.0 Earth masses.
  - Mars: 1.5 AU, 0.107 Earth masses.
  - Asteroid Belt: 2.3-3.3 AU, ~0.001 Earth masses. The largest asteroid is Ceres at 2.7 AU, ~0.000159 Earth masses.
  - Jupiter: 5.2 AU, 318 Earth masses.
  - Saturn: 9.5 AU, 95 Earth masses.
  - Uranus: 14 AU, 14 Earth masses.
  - Neptune: 30 AU, 17 Earth masses.
  - Kuiper Belt: 30-50 AU, 0.3-30 Earth masses. Almost like a 2nd Asteroid Belt. Includes dwarf planets like the binary Pluto and Charon.
  - Oort Cloud: 50,000-100,000 AU = 0.8-1.6 ly, 5-100 Earth masses. Comets come from the Oort Cloud while comet-like "Centaurs" stay within the planetary orbits.
  - The Sun's solar winds goes as far as 130 AU.
Our Sun
- Age: 5 billion (5e9) years out of the usual 10 billion for a class G2 star
- Diameter: 1,392,000 Km = 109 Earths
- Mass: 1.9891e30 Kg = 332,950 Earths
- Surface Temperature: 5780 K
- Core Temperature: 15,000,000 (1.5e7) K
- Speed about the galactic core: 220 Km / s, i.e. one revolution takes 226,000,000 years.
Our Earth
- Age: 4.5 billion (4.5e9) years
- Equatorial Diameter: 12,756.28 Km ~7,926.385 miles
- Mass: 1 Earth = 5.97e24 Kg = 1.32e25 pounds = 6.58e21 tons.
- Longitude is marked by vertical lines that run north and south on the Earth.
  - One mnemonic: They are all of consistent length (hence long).
  - 0 degrees longitude (the prime meridian) runs through the old Royal Astronomical Observatory in Greenwich, England, just outside of London.
  - West of the prime meridian is "negative", while east is "positive".
- Latitude is marked by horizontal lines that run east and west on the earth. One mnemonic: The are all parallel (hence the double t's).
  - 0 degrees latitude (the equator) is midway between the north and south poles
  - North of the equator is "positive".
    - The Tropic of Cancer is at +23°27' North.
    - The Artic Circle is at +66°33' North.
  - South of the equator is "negative".
    - The Tropic of Capricorn is at -23°27' South.
    - The Antarctic Circle is at -66°33' South.
- Moon
  - Perigee = Closest distance: 0.0024 AU = 363,104 Km = 225,662 miles.
  - Mass: 0.0123 Earths = 7.345e22 Kg = 1.62e23 pounds = 8.10e19 tons.
  - Diameter: 0.273 Earths = 3476 Km = 2160 miles
  - Equatorial gravity: 0.1564 Earths = 1.622 m * s ^-2.
Our Atmosphere
- 1000+ Km = 620+ milese. Outer Space.
- 500 - 1000 Km = 310 - 620 miles. 2773 K = 2500 C = 4531 F at the top but gases are extremely sparse. The Exosphere is considered beyond the atmosphere. Composed primarily of Hydrogen and helium.
- 80 - 500 Km = 50 - 310 miles. 2273 K = 2000 C = 3631 F at the top but gases are sparse. The Thermosphere, aka Ionosphere, the "upper atmosphere". This layer is highly charged/ionized by solar radiation thus enabling radio waves to bounce off of this layer.
  - 122 Km. The level that reentry from orbit begins.
  - 100 Km = 62 miles. The Karman Line, the usual boundary between the atmosphere and Space. Aerodynamic surfaces no longer function.
  - 80 Km. The U.S. calls its travelers that go beyond this level "astronauts".
- 50 - 80 Km = 30 - 50 miles. 200 K = -73 C = -100 F at the top. The Mesosphere, part of the "middle atmosphere". Meteors burn up in this layer.
  - 80 Km = 50 miles. The upper boundary is the Mesopause.
- 7/17 - 50 Km = 4/10 - 30 miles. 270 K = -3 C = 26 F at the top. The Stratosphere, part of the "middle atmosphere".
  - 50 Km = 31 miles. The upper boundary is the Stratopause.
  - 45 Km = 28 miles. Ramjets no longer function.
  - 32 Km = 20 miles. Turbojets no longer function.
  - 24 - 40 Km = 15 - 40 miles. The Ozone Layer which absorbs UV radiation. This layer, if at 1 atm would only be a few mm thick.
  - 24 Km = 15 miles. Regular aircraft pressurization systems no longer function.
  - 20 Km = 12 miles. Bodily fluids spontaneously boil without pressurized environments.
- 0 - 7/17 Km = 0 - 4/10 miles. 220 K = -53 C = -64 F at the top. The Troposphere, "lower atmosphere", bulges at the equator. Has 75% of the atmosphere's mass. Contains what we call "weather" and the greenhouse effect.
  - 7/17 Km = 4/10 miles. The upper boundary is the Tropopause.
  - 14 Km = 8.7 miles = 45000 feet. Cruising height of Concordes or military jets.
  - 9 Km = 5.6 miles = 30000 feet. Cruising height of most commercial jets.
  - 5.3 Km = 3.3 miles. 50% of the atmosphere's mass is below this level.
  - 3 Km = 2 miles. Supplemental oxygen for travelers required.
  - 0 Km = 0 miles. Sea level
    - Composition at sea level.
      - Nitrogen 78.084%
      - Oxygen 20.946%
      - Water 1% average
      - Argon 0.934%
      - Carbon Dioxide 0.035%
      - Neon 0.002%
      - Helium 0.001%
      - Methane 0.0002%
      - Krypton 0.0001%
      - Hydrogen 0.00006%
    - Pressure at sea level: 101.8 KPa = 14.7 pounds/inch = 1 atm.
Our Moon
- Diameter: 3,476.2 Km = 0.273 Earths
- Mass: 7.347673e22 Kg = 0.012 Earths
- Semi-major Axis: 384,400 Km
- Orbital Period: 27 Days, 7 hours, 43.7 minutes
- Earth to Moon: 1.28 light seconds = 384,000 km = 238,000 miles.

Page Modified: (Hand noted: 2007-10-24 04:47:03Z) (Auto noted: 2010-12-24 22:58:49Z)