# 9_celestial_distances.pdf - Celestial Distances - Solar...

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The work of Copernicus and Kepler established therelative distancesbetween the planets.But their work could not establish theabsolutedistances.To establish absolute distances, astronomers had to measure one distance in the solarsystem directly.Estimates of the distance to Venus were made as Venus crossed the face of the Sun in 1761and 1769.Celestial Distances - Solar SystemThese slides are based on the information presented in the OpenStax textbook “Astronomy”.Download for free at
Today, radar is used to determine the distances to planets in our solar system.It provides adistance measure in units of light seconds, minutes, or hours.Radar can’t be used to measure the distance to the Sun.We use KIII to do that.For our solar system, the Astronomical Unit is the preferred distance unit: 1 AU = 149 597870 700 metres.Knowing the absolute distance scale of our solar system allows us to determine absolutedistances beyond it.Celestial Distances - Solar SystemThese slides are based on the information presented in the OpenStax textbook “Astronomy”.Download for free at
Suppose you are trying to measure the distance to a tree.Suppose also there is a mountainbehind that tree.You set up two observing stations some distance apart. That distance iscalled thebaseline.Celestial Distances - Triangulation in Space
The position of the tree (in the foreground) relative to the mountain (in the background) isdifferent from each end of the baseline.This apparent change in direction of the remote object due to a change in vantage point ofthe observer is calledparallax.Celestial Distances - Triangulation in Space
If the tree were farther away, the whole triangle would be longer and skinnier, and theparallax angle would be smaller.The smaller the parallax, the more distant the object we aremeasuring must be.Celestial Distances - Triangulation in Space
Unfortunately, nearly all astronomical objects are very far away. To measure their distancesrequires a very large baseline and highly precise angular measurements.The Moon is the only object near enough that its distance can be found fairly accurately withmeasurements made without a telescope.Ptolemy determined the distance to the Moon correctly to within a few percent. He used theturning Earth itself as a baseline, measuring the position of the Moon relative to the stars attwo different times of night.To measure the distances to the nearby stars, we use the Earth’s orbit around the Sun as abaseline.As Earth travels from one side of its orbit to the other, it provides us with a baseline of 2 AU,or about 300 million kilometres.Even still, the stars are far enough away for their parallax shift to be undetected by the nakedeye.

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Stellar classification