Days can either be measured using sidereal time or solar time.
Sidereal time is the time the Earth takes to complete one rotation on its axis as measured against the stars and is called a sidereal day, divided into 24 sidereal hours.
Solar time is the time the Earth takes to complete one rotation on its axis as measured against the Sun. As each day the Earth moves around the Sun by approximately 1°, for the Sun to return to the same position in the sky each day the Earth must rotate one extra degree. This takes around 4 minutes thus a solar day is 4 minutes longer than a sidereal day. This is usually expressed as one sidereal day = 23h 56m of solar time.
Solar time is the system of time measurement used all over the world therefore stars (which run on sidereal time) seem to have a shorter 'day' than the one we go by.
So any particular star will rise 4 minutes earlier each night. A star that rises at 2200 on Monday will rise at 2156 on Tuesday and 2152 on Wednesday, and so on. After 15 days, it will rise 1 hour earlier. A star chart that depicts the layout at 0100 on the first day of the month will also be accurate at midnight on the 15th and 2300 hours on the 30th.
