# Why is the period of a geostationary satellite not exactly 1440 minutes?

When reading about Astra satellites on Wikipedia (https://en.wikipedia.org/wiki/Astra_1KR), I saw that the period of the Astra 1KR satellite, positioned at 19.2° E, is 1,436.1 minutes (source: NORAD data).

That is 3.9 minutes short of a day (1440 minutes), how is that possible with the satellite still being geostationary?

• @Exocytosis It would be possible to make a satellite follow the mean solar day, but that would not make it appear in the sky in a fixed position relative to the sun, because the earth's orbit is elliptical. The difference between "time as defined by the sun's position in the sky" and the mean solar day is called the equation of time and it varies by about $\pm20$ minutes during the year. Sep 17 '20 at 15:26