What Controls When and Where the Sun Rises and Sets?
THE SUN RISES EACH MORNING and sets each evening, but at slightly different times from day to day. Also, the Sun does not rise or set in exactly the same direction every day, although the changes from day to day are so gradual as to be unnoticeable. Over the course of several months, however, we can notice significant changes in when and where sunrise and sunset occur. What accounts for these variations?
Why Does the Sun Rise and Set?
- At any moment in time, half of the Earth's surface area is sunlit, experiencing day, and half is in the darkness of night. The dashed line encircling the world separates the lighted and dark halves and is called the circle of illumination. When viewed straight on, as in these figures, the circle of illumination appears as a line, but it has a curved shape from any other perspective. The colored areas on these globes are time zones.
- In the left globe, North America and western South America are on the side of the Earth hidden from the Sun, and so it is night. At the same time, eastern South America and Africa are on the side facing the Sun and so are in daylight. With a rotating Earth, this view is only an instantaneous snapshot of which areas are in sunlight and which are in darkness.
- In the right globe, the Earth has rotated an additional four hours — the globe rotates to the right when viewed in this perspective, or counterclockwise when viewed from above the North Pole. At this later time, South America and eastern North America have rotated into the sunlight (it is morning), but the western U.S. is still in the last remaining hours of night.
- The circle of illumination, the boundary between day and night, moves westward across the surface as the planet rotates eastward. As this occurs, the Sun has not changed position — the Earth has simply rotated. It finishes a complete rotation in 24 hours.
Why Does the Length of Daylight Vary Through the Year?
In most parts of the world, there are significant differences between the length of daylight from season to season. In such regions, days are noticeably shorter during the winter than during the summer. In accordance with this, nights are longer during the winter and shorter during the summer. In contrast, at the equator, there are always 12 hours of day and 12 hours of night, irrespective of the time of year.
- These globes show the circle of illumination at three different times of year: the December Solstice, an equinox, and the June Solstice. To help us visualize the circle of illumination, the three larger globes are depicted as if being observed by the small figure next to the corresponding small globes. The axial tilt remains fixed in orientation as Earth orbits the Sun, but here it is portrayed from different perspectives.
- During an equinox (the large, center globe), the tilt axis is oriented neither toward nor away from the Sun. So, the pattern of light and dark is symmetrical with respect to the equator and other lines of reference. It takes the same amount of time for every location to rotate in and out of sunlight. At equinox, every location on Earth has 12 hours of sunlight and darkness.
- At other times of the year, Earth's axis appears tilted toward or away from the Sun, and so the circle of illumination is not symmetrical relative to lines of reference, such as the tropics. In the left globe, representing the December Solstice, any line of latitude in the Southern Hemisphere, such as the Tropic of Capricorn, is more in sunlight than in darkness. Therefore, days are longer and nights are shorter. The opposite is true for any latitude in the Northern Hemisphere, which is more in the dark than in the light, causing nights to be longer than days.
- During the June Solstice and adjacent months, the opposite is true — more of the Northern Hemisphere is in sunlight than is in darkness. As a result, days are longer than nights in the Northern Hemisphere during this time (the northern summer). In contrast, more of the Southern Hemisphere is in darkness, so days are shorter and nights are longer during this time (the southern winter).
- The variation in the lengths of day versus night from season to season increases with increasing latitude, either north or south of the equator. Such seasonal changes in day length are absent at the equator and greatest at the poles.
Why Do Arctic Areas Sometimes Have 24 Hours of Sunlight or Darkness?
The most extreme variations in the lengths of daytime and nighttime occur in the highest latitudes, including the Arctic region around the North Pole and the Antarctic region around the South Pole. North of the Arctic Circle and south of the Antarctic Circle, summer days can have 24 hours of straight daylight. During the winter, it can remain dark for all 24 hours, night after night. Either condition can last for months. How is this so?
This image combines different photographs to show the path of the Sun during several hours at a location north of the Arctic Circle. The Sun remains low in the sky and dips toward the horizon at midnight, but never actually sets — this location has 24 hours of sunlight during the middle of summer. The low Sun means that insolation striking the land is spread out and so is relatively weak, and it has a long and attenuated path through the air.
The figure above shows sunlight on the north polar region during the December Solstice. All the area within the Arctic Circle is in darkness (it is on the side opposite to the Sun). As the Earth rotates about its axis, the entire area within the circle remains out of the sunlight — 24 hours of darkness. In days following the Solstice, sunlight begins to creep into the Arctic Circle, so less of the Arctic has 24-hour nights.
The opposite situation occurs during the June Solstice, shown here. Note that the entire Arctic Circle faces the Sun and will remain in sunlight as the Earth rotates about its axis. On days before and after the solstice (when the North Pole less directly faces the Sun), areas just inside the Arctic Circle would not be in constant sunlight, and so would have slightly less than 24 hours of sunlight and minutes to hours of nighttime.
What Controls the Time and Direction of Sunrise and Sunset?
Except at the equator, the times of sunrise and sunset shift slightly from day to day, but typically by less than a few minutes every day. From month to month, however, we notice significant differences in the times of sunrise and sunset, and therefore in the duration of day and night. The changes in time are accompanied by gradual changes in the direction from which the Sun rises and the direction in which it sets.
- This figure depicts where and when the Sun rises and sets at a location at 45° N latitude, which is halfway between the equator and the North Pole. Observe this figure and note how the locations of sunrise and sunset change by date.
- The December and June dates, marking the two solstices, show the two extremes. The locations and times of sunrise and sunset fall between these two extremes for all other dates. Either equinox is halfway between the solstices. Each day, the Sun's path defines a circle, which lies on a plane that is inclined at an angle that is equal to the site's latitude (45° in this case).
- At the December Solstice, the Northern Hemisphere faces away from the Sun the maximum amount, and so the Sun is as low in the sky as it ever gets. The Sun rises and sets as far south as any day of the year, rising in the southeast and setting in the southwest. This is the shortest day and longest night of the year.
- At either equinox, neither pole faces the Sun, and so there are 12 hours of daylight and 12 hours of darkness at every location on Earth. Everywhere on Earth, the Sun rises due east of the site and sets due west on an equinox. If you wanted to determine directions without a compass, and it happened to be the date of an equinox, you could precisely determine an east-west direction by drawing a line from the direction of sunrise to the direction of sunset, as was done by ancient cultures. Also, you could determine your latitude by measuring the zenith angle at noon on this day—your latitude is the zenith angle.
- At the June Solstice, the Northern Hemisphere faces the Sun, and so the Sun is the highest in the sky it will be all year. The Sun rises and sets as far north as any day of the year, rising in the northeast and setting in the northwest. This is the longest day and the shortest night of the year.