Earth Science - Other

The Impact of Latitude on the Length of a Day

Tweet
Rex Trulove's image for:
"The Impact of Latitude on the Length of a Day"
Caption: Latitude lines
Location:
Image by: National Atlas of the United States

People have long noted that the farther away they move from the equator, the greater the variation is between the time of sunrise and sunset. Many people believe that this variation has been used to figure out the latitude of a given location, and it is probable that ancient mariners used this fluctuation to figure out their northerly or southerly positions, long before the reason for the variation had been explained.

Latitude

The latitude is a reference to how far north or south of the equator a person or location is, on the surface of the earth. Thus, the equator is denoted as 0 degrees north or south. The north pole would be 90 degrees north, while the south pole is 90 degrees south. Between the two, latitudes can be thought of as imaginary circles around the sphere of the earth. This is over-simplified, however it is important in understanding what the latitude has to do with sunrise and sunset.

Earth's revolution

The earth revolves on its axis, an imaginary line passing through both the north and south poles, once per day. It is this that gives the appearance of a sunrise or sunset. This can be visualized by shining a light on a rotating ball, where a point on the ball passes alternately from light to darkness to light again. It can also be seen that if the light is shining directly on the midpoint of the ball, the time of 'sunrise' and 'sunset' will be equal with each revolution, assuming that the ball is revolving at a constant speed.

This also means that the farther toward the top or bottom of the ball a point is, the greater the difference will be between periods of light and periods of darkness. A point on the middle of the ball will have equal periods of light and darkness while the top and bottom of the ball will be in near darkness all the time. This means that the greater the latitude, the more darkness a point on Earth can expect to get.

Earth's tilt

The axis of the earth is tilted by about 23.5 degrees in relation to its orbit. This not only is a major cause of seasons, it also has a great impact on sunrise and sunset, particularly at different latitudes. Using the previous example, tilt the ball 23.5 degrees and this is easy to see. The ball is rotating on its axis while also revolving around the light, so part of the time the top of the ball is in light while the bottom is in darkness. As it continues around the light, this changes until this is reversed and the top of the ball is in darkness while the bottom is in light. This defines the seasons and has a direct relation to latitude.

Latitude and tilt

On the earth, the tilt is responsible for the phenomenon of having six months of short days, followed by six months of short nights at the poles, each year. For each latitudinal point closer to the equator, as can be seen in the model that has been used here, the difference in time between light and darkness gets smaller. Finally, at the equator, day and night are the same length of time, regardless of where the earth is in its orbit around the sun.

Again, this is simplified and doesn't take into consideration such things as the earth's 'wobble'. However, it does make it easy to see that a north or south latitude has an enormous impact on the time of sunrise or sunset. The greater the latitude, the greater the variation usually is. This also explains why a place in Florida will have less variation in the amount of time between sunrise and sunset than a place in Maine will, on the same day, excluding equinoxes, for instance. NOAA (National Oceanographic and Atmospheric Administration) has even designed a handy calculator to figure out the time of sunset and sunrise based on latitude.

Tweet