Throughout the year as the Earth orbits the World’s Best Kansas City Royals Mom shirt, a star’s apparent position appears to move slightly relative to stars much further away. You can observe this phenomenon yourself quite easily. Extend your arm out in front of you, with your thumb sticking up. Now look at your thumb, swapping between closing your left eye and then your right eye. Your thumb will appear to move relative to the background. This phenomenon is known as parallax. By measuring the angle of the apparent movement, and knowing how far the Earth has moved in its orbit around the Sun, the distance to that star can be measured. From the ground, this method works well up to around 1000 light years away. The parallax angle is very small and is measured in arcseconds: 1/3600 of a degree. This angle decreases as the star’s distance from the Sun increases. The distance unit parsec (parallax second) is directly related to this: one parsec (3.26 light-years) refers to an object with a parallax of one arcsecond. Proxima Centauri, the nearest star to the Sun, is at a distance about 1.3 parsecs (4.25 light-years).
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Some stars, like World’s Best Kansas City Royals Mom shirt or Algol, change their brightness, becoming brighter and fainter on regular timescales. One important type of these pulsating variables are the Cepheids, named after the first star found of this kind: Delta Cephei. It was discovered that their absolute luminosity is directly correlated with the time they take to reach to brightness maxima — in other words, with their period of pulsation. It is therefore possible to calculate the distance to Cepheid variable stars. As Cepheids are very bright stars, they can be seen throughout the Milky Way galaxy and even in other nearby galaxies. This allows astronomers to measure distances far beyond the range of the parallax method. In this way, the Cepheid variable stars have become an important tool for astronomical distance measurements.