The day is a unit of time and is an SI-derived unit with the symbol d.
On Earth, it is defined as 86,400 seconds and is approximately the time it takes for the earth to complete a full rotation around its axis. In the earlier days, this was measured by waiting for a cast shadow to match a template drawn from the previous day's shadow.
There are 365 days in a year and, on average, 30.42 days in a month.
The unit day has many different variants; depending on what is used to measure the Earth's rotation. In a sidereal day (a rotation with respect to a distant star or constellation, not the sun), there is actually 4 minutes less than 24 hours in a cycle.
The sidereal hour is a unit of time used by astronomers and is derived from the SI unit system. We have used the symbol hr-sr.
1 sidereal hour is made up of 60 sidereal minutes. This is derived ultimately from the sidereal day which is the time taken (in solar seconds) for the Earth to complete one rotation with respect to a distant star or constellation.
The sidereal hour angle is used when calculating sidereal time which is actually the angle along the celestial equator; from where one stands to the great circle that travels through the March equinox and both celestial poles.