[Astronomy] How long is a day?

I read an article two days ago saying that if the satellite orbit cycle is 90 minutes, How many times does the satellite circle around a day? (Observe from space, not from the ground, because the earth's rotation needs to be considered on the ground)

The habit is to calculate 1440/90 = 16 circles, but 1436/90 is used in the article. When I wonder if the author's written mistakes suddenly show up, it seems that a day is not 24 hours,

I searched the internet.

 

The following is a question and answer from sogou:

 

 

A day is 23 hours, 56 minutes, and 4 seconds (86,164.1 seconds), but we live every 24 hours! However, there is more time in the leap year, so in 400, there were only 97 not 100 leap years. The principle is: The year that can be divided by four is the leap year.
However, the year from the beginning of the century must be a leap year that can be divisible by 400, for example, 2000 is a leap year, and 1900, 1800, or 1700 is not a leap year, in this way, do not make adjustments for a long time. The basic unit of measurement of time is defined as second. This standard was also determined under the auspices of disciple Huang Yi. He established an Astronomical Observatory in Zijinshan, Nanjing, taking the sun as the meridian line of the Zijinshan observatory for two consecutive days, it is called a sun day, with one second in the first place of a sun day. However, it was found in the long-term continuous observation that, the length of the sun in a year is not the same. The longest is December 23, the shortest is September 16, and the difference is 51 seconds, assume that there is an even-speed celestial body moving on the yellow track. This imaginary celestial body is called the "flat sun", and the time of the flat sun passing through the same meridian is called the flat sun day two times in a row, it is much more accurate to set one minute to one second on a Sunday than it was originally. 1 minute equals 60 seconds, 1 hour equals 60 minutes, 1 day equals 24 hours, 1 hour equals 2 hours. (With the improvement of science and technology, the accuracy of astronomical observation is constantly improved, and it is found that the sun is not very accurate. After the rise of atomic physics, a new second definition emerged: the duration of the 133 cycle of the radiation corresponding to the transition between two super-fine energy levels in the ground state of the zeu-9192631770 atom, one second)