Today I am posting from a remote site which some how seems appropriate since today's topic is on astronomical measurements. The first thing to consider when talking about anything in space is the distance between two objects is usually insanely far apart.
Within the Solar system we usually stick to miles or kilometers. Even the far out ice ball called Neptune is measured in km all 4,553,946,490 of them. Now that is a whole lot of kilometers to be sure. the real problem is to get that distance to a manageable number. Enter the AU or Astronomical unit. This unit has been many different Values throughout history. Aristarchus of Samos Greece had first stab at it. He said it was about 20 times the distance to the Moon. the ratio is about 390 times! He was wrong about that distance but he was right about a heliocentric system. Ptolemy by the second century had the distance at about 20 radii of the earth. We now have come to the conclusion and standardized by the IAU the distance of149,597,871 km (92,960,000 miles) to be the winning number Using this AU value Neptune at 4.5 billion and change converts to 30.1 AU. Ahhh much better.
- The Moon is 0.0026 ± 0.0001 AU from the Earth
- The Earth is 1.00 ± 0.02 AU from the Sun
- Mars is 1.52 ± 0.14 AU from the Sun
- Jupiter is 5.20 ± 0.05 AU from the Sun
- Pluto is 39.5 ± 9.8 AU from the Sun
- The Kuiper belt begins at roughly 35 AU
- Beginning of Scattered Disc at 45 AU (10 AU overlap with Kuiper Belt)
- Ending of Kuiper Belt at 50-55 AU
- 94 AU:Termination shock between Solar winds/Interstellar winds/interstellar medium
- 100 AU:Heliosheath
- 110 AU: As of June 2009, Voyager1 is the furthest of any human-made objects from the Sun: it is currently travelling at about 3½ AU/yr
- 100-1000 AU: Mostly populated by objects from the Scattered Disc
- 1000-3000 AU: Beginning of Hills cloud/"Inner Ort cloud"
- 20,000 AU: Ending of Hills Cloud/"Inner Oort Cloud", beginning of "Outer Oort Cloud"
- 50,000 AU: possible closest estimate of the "Outer Oort Cloud" limits (0.8 ly)
- 100,000 AU: possible farthest estimate of the "Outer Oort Cloud" limits (1.6 ly)
- 125,000 AU: maximum extent of influence of the Sun's Gravitational field (Hill Roche sphere). beyond this is trueInterstellar medium. This distance is roughly 1.8-2.0 light-years
- Proxima Centari(the nearest star to Earth, excluding our own Sun) is ~268 000 AU away from the Sun
- The mean diameter of Betelgeuse is 5.5 AU (822 800 000 km)
- The distance from the Sun to the centre of the Milky way is approximately 1.7 × 109 AU
But even the AU has its limits. When we talk about how far a star or even better a galaxy is to Earth, the AU starts looking like those numbers followed by a thousand Zeros! well not quite that bad but you get the picture. BIG! The answer was hidden in the speed of light. Light travels at 186,000(approx) miles a second. so after one year of travel how far could you go? How about about 5,878,630,000,000 miles or so. Just about 6 trillion miles equals 1 light year. that sounds pretty doable as far as distance to the way out stuff. The light year was hardly embraced by the science community.
But hang on what about the Really far out stuff that made the Light year value look crazily huge?Enter the Parsec. the parsec arrives at its figure using the parallax of the Sun- Earth and distant objects through trigonometry. The Parsec has a value of 3.26 Light years. Well that helps a bit more to get some of those Galaxies that are so far out a little more manageable. I know this is a short post but Sometime you just have to drive two hours on no notice to shovel snow off your parents drive. I will post more tomorrow
Until then ,
Clear skies and great seeing too