James Echeverria, age 13, of Santa Rosa, Calif., for his question:
WHAT DO A.M. AND P.M. STAND FOR?
A.M. stands for the Latin words ante meridiem, which means “before noon.” P.M. stands for the Latin words post meridiem, which means “afternoon.” There are 12 A.M. hours and there are 12 p.m. hours. Put them both together and you have one day.
Each day begins at midnight. The hours from midnight to noon are the a.m. hours while the hours from noon to midnight are the p.m. hours.
Not everyone started his day at midnight. The Babylonians began their day at sunrise while the ancient Jews begin their day at sunset. The Egyptians and the Romans were the first to begin the day at midnight.
A solar day is the length of time that it takes the earth to turn around once with respect to the sun. We usually say “day” for the time when the sun is shining on our part of the earth, and “night” for the time when our part of the earth is dark, or turned away from the sun.
But, of course, the night is really apart of the whole day.
People in the military services often designate the time of day on a 24 hour basis. They say 0100 for one o’clock in the morning. Noon is called 1200 and when midnight comes around, it is 2400.
The length of daylight changes during the year in all parts of the world. The reason is because the tilt of the earth’s axis causes first one pole to slant toward the sun and then the other as the planet orbits the sun.
The shortest day of the year in the Northern Hemisphere usually is December 21 while the shortest is the Southern Hemisphere is usually June 21. It is just the other way around for the longest day: in the Northern hemisphere it usually falls on June 21 while in the Southern Hemisphere it is usually December 21.
The first day of winter falls on the shortest day while the first day of summer is on the longest.
The longest day of the year has 13 hours and 13 minutes of daylight at 20 degrees latitude. The same day has 14 hours and 30 minutes of daylight at 40 degrees latitude and 18 hours and 30 minutes at 60 degrees.
The shortest day has 10 hours and 47 minutes of daylight at 20 degrees latitude nine hours and nine minutes at 40 degrees and only five hours and 30 minutes at 60 degrees.
Astronomers use a day called a sidereal day. It is based on the period of the earth’s rotation as measured by fixed stars. This day equals 23 hours, 56 minuts and 4.09054 seconds of mean solar time.
When the tilt of the earth’s axis causes the North Pole to face the sun, the South Pole is continuously dark and the North Pole is always in daylight. As the North Pole is tilted away from the sun, it becomes dark there while the South Pole has constant daylight. These periods of darkness and daylight last about six months.