The Winter Solstice usually occurs around December 21 in the Northern Hemisphere and June 21 in the Southern Hemisphere. Many believe that the year’s latest sunrise and earliest sunset also happen on this day. This is not the case.
If you look at the sunrise and sunset timings for any city in the Northern Hemisphere around the December Solstice, you will notice that the earliest sunset occurs a few days before the Solstice. Similarly, the latest sunrise happens a few days after the solstice.
This is also true for locations in Southern Hemisphere. The year's earliest sunset in the Southern Hemisphere happens a few days before the Winter Solstice in June, and the year's latest sunrise occurs a few days after the Solstice.
An example
Let’s take the cases of New York City, United States and Sydney, Australia.
New York City is located at 40°43' North latitude. In 2012, the Northern Hemisphere Winter Solstice occurred on December 21, 2012. The same year, New York City experienced its earliest sunset on December 6, 2012 and the latest sunrise on January 10, 2013.
Sydney, Australia which is in the Southern Hemisphere and is located at 33° 8' South, experienced its latest sunrise of 2013 on July 6, and the earliest sunset on June 5, 2013. The Winter Solstice for the Southern Hemisphere occurred on June 21, 2013.
What explains this curious occurance? Why doesn't the shortest day of the year also have the latest sunrise and the earliest sunset of the year?
Two different factors combine to cause this interesting phenomenon:
- the equation of time
- a location's latitude.
The Equation of Time
Very simply, the equation of time is the difference between time that is measured using a sundial (true or apparent solar time) and time that is measured using a watch or a clock (mean solar time).
Most clocks work on the idea that a day - the time between one noon to the next - is exactly 24 hours.
Scientifically, however, a day is defined as the duration between 2 solar noons. A solar noon is the time of the day when the Sun is at the highest point in the sky, and a solar day is the duration between two solar noons.
A solar day is not exactly 24 hours long. Its length varies throughout the year. In fact, the solar day is longer than 24 hours around the summer and winter solstices and is shorter than 24 hours around the spring and fall (autumn) equinox. This means that the length of the solar day does not always match up to the length of a day as measured by a clock. This is because of two reasons:
Elliptical shape of Earth's path around the Sun
The shape of the Earth’s orbit around the Sun is closer to an oval than a perfect circle. The technical term for its shape is an ellipse. The Sun is not situated in the center of this elliptical orbit, but closer to one side than the other. Because of this, the Earth comes closer to the Sun(the Perihelion) during one part of its orbit than the other (the Aphelion).The speed at which the Earth moves around the Sun varies throughout the year. When it is closest to the Sun, the Earth travels about 13,500 miles (21,726 kms) more each day than when it is away from the Sun on its orbit.The Earth's axial tilt
In addition to moving around the Sun, the Earth also rotates on its own axis. The axis is an imaginary line that passes through the North and the South pole. It is tilted to about 23.5° in relation to the Earth's orbital plane around the Sun (the ecliptic). This tilt is known as the Earth'sobliquity, and is responsible for seasons.
The combination of these two forces leads to changing lengths of solar day during the year. This means that on most days the solar noon will not occur at the same time as noon on your watch. The equation of time or the difference between apparent solar time and mean solar time is about +14 minutes in February and around -16 minutes in October.
Equation of time is often pictorially represented by a figure 8, also known as the Analemma.
Location's Latitude
The dates for the earliest sunset and latest sunrise for a location also depend on its latitude. Locations closer to the equator have their earliest sunset sometime in November. Locations at higher latitudes, on the other hand have their earliest sunset later, closer to the actual date of the winter solstice.
This occurs because of the Sun’s declination. The Sun's declination is the angle at which the rays of the Sun hit the plane of Earth’s Equator.
Because the Earth’s rotation axis with respect to the Sun is tilted to about 23.5°, the Sun’s declination varies through the year. During the summer solstice it is +23.5° and during the winter solstice it is -23.5°. It is 0° during the equinoxes.
During the equinoxes, when the Sun’s declination is 0°, most locations on Earth, with the exception of locations on and around the North and the South Pole, experience around 12 hours of sunlight. On the other hand, when the solar declination is +23.5°, all locations in the Northern Hemisphere experience over 12 hours of daylight, while the Southern Hemisphere experiences less than 12 hours of daylight. This effect reverses when the sun’s declination is -23.5°.
This varying lengths of daylight through the year due to the declination of the Sun also affect the timings of sunsets and sunrises.
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