The Equator and Prime Meridian are circles that stretch all the way around the Earth.
The Equator separates the Northern and Southern Hemispheres. The Equator is at 0° latitude.
The Prime Meridian separates the Eastern and Western Hemispheres. The Prime Meridian runs through Greenwich, England and is at 0° longitude.
Day length at the Equator
At latitude 0° (the Equator) day length will be approximately 12 hours. Areas on the Equator have a constant 12 hours of day light all year round.
Moving away from the Equator
As latitude increases to 80° (polar circles - north or south) day length can be seen to increase to 24 hours or decrease to zero (depending on time of year).
In summer, the regions north of the Arctic Circle enjoy between one and two months of Midnight Sun – 24 hours of sunlight due to the relative tilt of the Earth towards the Sun.
It is the opposite below the Antarctic circle (South Pole).
In winter, the regions north of the Arctic Circle experience a 24-hour period when the sun never rises above the horizon. This is known as the Polar Night. Again it is the opposite below the Antarctic circle (South Pole).
Follow this link to read more about the Prime Meridian