When it is daylight on the half of the Earth facing towards the Sun, it is night on the half of the Earth facing away from it. As the Earth rotates, so the day and night halves shift gradually around the world.
The Earth turns eastwards — this means that the Sun comes up in the east as our part of the world spins round to face it.
As the Earth turns, the stars come back to the same place in the night sky every 23 hours, 56 minutes and 4.09 seconds. This is called a sidereal day (star day).
It takes 24 hours for the Sun to come back to the same place in the daytime sky. This is the solar day, and it is slightly longer than the star day because the Earth moves one degree further round the Sun each day.
On the other planets, the length of day and night varies according to how fast each planet rotates.
One day on Mercury lasts 59 Earth days, because Mercury takes almost two months to spin around.
A day on Jupiter lasts less than 10 hours because Jupiter spins so fast.
A day on Mars is 24.6 hours — much the same as ours.