The following sun and moon data for Dec. 18, 2008 is provided by the United States Naval Observatory.

Sunrise: 7:17 a.m.

Sunset: 4:53 p.m.

Moonrise: 10:56 p.m. Dec. 17.

Moonset: 11:35 a.m. Dec. 18.

Moon phase: The moon is waning gibbous with 57 percent of the visible disk illuminated. The moon is at last quarter Dec. 19 at 3:30 a.m. Mountain Standard Time.

Winter officially begins Sunday morning, Dec. 21, at 5:04 a.m. with the Winter Solstice.

In addition to marking the start of winter, the solstice — Latin for “sun stands still” — also marks the shortest day of the year. From Sunday forward, the sun gains altitude in our sky as winter begins for the Northern Hemisphere and southern hemisphere residents experience the start of summer and their longest day of the year.

Although Dec. 21 marks the shortest day of the year for the Northern Hemisphere, it does not mark the day of the earliest sunset. According to the United States Naval Observatory, the earliest sunset occurred Dec. 7 for most of the Northern Hemisphere.

The confusion comes when stargazers focus on clock-based sunset and sunset times rather than true solar noon. True solar noon is the time of day when the sun reaches its highest point as it traverses the daytime sky.

In early December, true solar noon comes several minutes earlier by the clock than it does at the solstice around Dec. 21. With true noon coming later on the solstice, so do the sunrise and sunset times.

The discrepancy between clock time and sun time causes the earliest sunset and the earliest sunrise to precede the December solstice. However, the date of the earliest sunset also depends on one’s latitude. At mid-northern latitudes, such as Colorado, the earliest sunset comes in early December each year. Farther north, in places such as Canada and Alaska, the year’s earliest sunset comes around mid–December. As you reach the Arctic Circle, the earliest sunset and the December solstice occur on or near the same day.

In addition, the latest sunrise does not come on the solstice either — for mid–northern latitudes, the latest sunrise comes in early January. Thus, and although the dates vary, the sequence is always the same: earliest sunset in early December, shortest day on the solstice around Dec. 21, latest sunrise in early January.

What causes the solstice?

The solstice is caused by the Earth’s tilt on its axis and its motion in orbit around the sun. Because Earth does not orbit upright, but is instead tilted on its axis by 23–and–a–half degrees, Earth’s northern and southern hemispheres trade places in receiving the sun’s light and warmth most directly. Thus, at the December solstice, Earth is positioned in its orbit so that the North Pole is leaning 23–and–a–half degrees away from the sun, while on the Summer Solstice that tilt is reversed.