Montana Earth Science Picture of the Week

View of Earth during the Winter Solstice

The Sun stops (not really) . . .
This view of Earth helps depicts the Earth on the Winter Solstice (around December 21 each year). It shows why days are so short and nights so long this time of year. To fully understand the shortness of our days, here's a few things to keep in mind:

1. Earth orbits the Sun once every 365.25 days.

2. The Earth's axis is tilted 23.5 degrees* (see NOTE below). If it weren't, there would be no seasons and every place in the world would have 12 hours of daylight and 12 hours of darkness every day of the year. A common misconception about the seasons is that they are caused by Earth being closer or farther from the Sun at different times of the year. THAT IS NOT CORRECT. In fact the Earth is about 3 million miles closer to the Sun in early January than it is in early July! The average distance from the Earth to the Sun is 93 million miles. Here's an animation that may help, but keep in mind that the Sun is MUCH bigger than Earth.

3. The Earth spins on its axis once every 24 hours. This axis runs through the Earth from the North Pole to South Pole.

4. Around December 21, Earth reaches the position on its orbital path where the Northern Hemisphere is tilted away from the Sun. Six months from now, Earth will be on the opposite side of the Sun and our hemisphere will be "leaning" toward the Sun.

Good news and bad news . . .
As shown on the image, on December 21, residents of Helena (star) could have observed sunrise at 8:09 AM. (The large yellow circle shows the path of Helena during the 24 hours it takes Earth to spin on its axis.) As Earth continued to spin, Helena moved into darkness (sunset) at 4:43 PM. So, on this, the "shortest day" of the year, Helena experienced only 7.5 hours of daylight followed by 16.5 hours of darkness. In comparison, Anchorage, Alaska (A) would have experienced about 6 hours of daylight (sunrise at 10:14 AM, sunset at 3:41 PM). The good news is that since December 21, the amount time that Montana gets to spend in the Sun has been increasing by minutes per day. This trend will continue until mid-June. CLICK HERE to get sunrise and sunset times for your city.

NOTE: The term "solstice" means "Sun stops." This term is used for December 21 because this is the day that the Sun stops getting lower in the sky (also directly related to Earth's tilt and the position of Earth on its journey around the Sun). This day is also the first official day of winter.

*NOTE: The tilt of Earth's axis does change slightly over tens of thousands of years, but the change during the course of one year is insignificant.

Term: equinox


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By Rod Benson
Earth Science Teacher at Helena High School

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