Reality → Scale → Macro world → Astronomical distances
How to determine astronomical distances
- Solar system. Third Keppler law (applies to all planets): square of planet's rotation period around
Sun is proportional to cube of distance from Sun. This
allows to determine the relative sizes of all planet orbits. In 1672, Richer
and Cassini determined the distance between
Earth and Mars by measuring the parallax of Mars (1/100 degree) from
Cayenne and Paris (baseline distance about 10,000 km). All absolute distances in the solar system then followed.
- Near stars. Using the diameter of Earth's orbit as the baseline for determining the parallax of
nearby stars (I.e. observing their shifted position on celestial images taken in summer and winter, determining half the
angle between these two positions and then calculating the distance by triangulation).
Telescopes on Earth can measure angles down to 0.01 arcseconds, equivalent to about 300 lightyears distance, satellite telescopes can
extend the distance by a factor of 100.
- Our galaxy and beyond. Distance can be calculated from apparent
luminosity if real luminosity is known (luminosity decreases with the square of distance). Stars of same type are assumed to have
same real luminosity and are used as standard candles.
Variable stars (twin stars circling each other at short distance
and thereby causing periodic light changes) and supernovae type
1a (exploding white
dwarf stars) are common candles to determine distances beyond our galaxy. Spectral analysis allows classification of stars through
estimation of their elements, surface temperature, and real luminosity. Estimates of distance to most remote galaxies are derived from
the expansion of the universe (see Spectroscopy and Cosmic motions).
← Macro world