The Astronomical Unit

On my post of my birthday, I mentioned the average radius of the orbit of the Earth. Of course, I bet most of you educated people know that Earth does not orbit in circle. To those who don’t know, it orbits in an ellipse, which is basically like an oval. The distance from the Earth to the sun differs, but on average, it is 149.6 millions of kilometes. That is what astronomers use as what is called the astronomical unit. Space itself is so large that astronomers need a unit to represent millions of kilometers. Even then, it is useful only for the distance of a stellar system, since farther measurements requires the parsec or light years. Well, you could use the astro unit, but the number would be freaking huge. Perhaps it is useful if you put the prefix mega or kilo or peta or giga or…. etc.

(Transitioning to the unreliable and disgusting to calculate English system of measurement)

The astronomical unit was originally a placeholder for the distance of the Earth to the Sun. It was useful in determining how far other planets were compared to the Earth. Copernicus and Kepler were actually very close at determining the distances in AU to the modern value, but the actual lenghts were estimates at best. Originally, A Greek’s universe was small because Earth was at its center, and planets would have had to go around it really quickly. Then came the realization that the sun was the center, (actually, on the focus of an ellipse) and Earth’s orbit was thought out to be really small compared to the values today. Kepler’s estimate estimate of 4.8 million miles was HUGE, but not huge enough. The mind does not comprehend how vast this freakishly mind boggling place is.

The first method they came up with was micrometry, which measured the diameter of the planet as seen on Earth. Then one guessed the size of the planet, say, Mars, being 60% the size of the Earth (modern values is 53%), and then make calculations to measure the distance. One scientist, Huygens did it, and he came up with 100,000,000 miles. That was a little bit too huge, since the real value was 93 million miles. It is close enough, though. The problem was that Huygens had to guess the correct value of Mars. In science, guesses and speculations had to be proven, and in this case, no one knew the planet’s actual size, so, oh well, too bad Huygens.

To do valid measurements, they had to use a technique called the parallax. Basically, one measures the position of a planet from one side of the Earth, and another person measures it from another place. The position of the planets would look different relative to the background stars. By measuring the angles of such changes, and using a bit of trigonometry, one could find the distance of the planet. Here is a picture of how parallax works:

There was a problem, though. The problem was measuring the latitude, and the clocks were not helping, having to synchronize them so that astronomers make their observations all at the same time. Knowing Earth’s circumference, though, was accomplished Giovanni Cassini and Jean Picard, and used an ingenious method of Galileo to measure Earth by observing Jupiter’s moon, and the result was accurate within 200 miles. With good clocks and maps, though, the first attempt at the parallax was in 1672, done with Mars by astronomer Jean Richer and his colleague. It went reasonably close, but a bit to the smallish side of 87 mill miles, close to the figure of 93. Unfortunately, innacuracy of the instruments meant that it was still not fully right.

An earlier attempt was with Mercury and Venus. Since it is closer than Mars, triangulation was more accurate. The bad part, though, was that it HAD to be done when Mercury and Venus was the closest to Earth, which meant that the sun’s glare would block the sight. Therefore, observations had to be done during transits, when Mercury and Venus went in front of the sun, making them seem dark in the background of the shining sun. The earlier attempts, though, failed. In the Mercury one, an assistant got impatient and wandered off. On the pair of Venus transits in 1631 and 139, things didn’t go smoothly either. In the first one, it was only visible in America, and in the second one, they weren’t ready to measure the angular diameter of Venus.

By the 1761 and 1769 (they waited a century to triangulate!), a bunch of astronomers were sent around the world and sacrificed their well being to measure it. The final result? Under James Cook and Joseph Banks, they came within 10% (9,300,000 miles) of the correct distance, and as time passed, precise measurement yielded: *drum rolls* 93 million miles!

In my mind, the finding of the astronomical unit was a monumental part in science and human history. With the measurement of the AU, it was the beggining of  the realization that the universe was large, and large as in: incomprehensively mind blowing, brain splattering, and ear drum shattering large, and the fact that the whole universe doesn’t revolve around us. Oh, but that was only the beggining. The AU was nothing, and I mean, NOTHING, compared to the size of this galaxy, and even the whole universe. That realization, people, is nothing short of amazing.


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