Washington State University
Dr Universe: Why does the sky darken in winter? – Alex, 6, Cincinnati
As winter begins here in North America, you may notice that we don’t feel the sun’s rays for as many hours as we do in the fall and summer.
To find out why this is happening, I spoke with my friend Vivienne Baldassare, an astronomer at Washington State University.
She said the reason we have fewer hours of daylight in winter is related to the Earth’s rotation. When our planet revolves around the sun, it is always in rotation. This rotation is also the reason why we have day and night.
When the part of the Earth you are standing on faces the sun, it is daylight. When the part you are standing on is away from the sun, it is dark.
But the reason the length of daylight changes throughout the year is because our planet is tilted a bit.
âThe Earth is not perfectly straight from top to bottom,â Baldassare said. “It’s tilted a bit on its axis, more like a spinning top.”
The top half of the planet, known as the northern hemisphere, slopes more towards the sun in summer. But when it’s winter, it means the northern hemisphere of the Earth is tilted away from the sun. It can make the days shorter and the shortest day of the year is fast approaching.
In Spokane we will have 8 hours 25 minutes of daylight on Tuesday. It is the day we mark as the winter solstice when the Earth’s north pole reaches its maximum tilt relative to the sun. You may also know it as the first day of winter.
After the winter solstice, the days will start to lengthen again in most places. But there are places on our planet, like TromsÃ¸, Norway, that will be completely dark for a few months.
âThe further north you go, the sooner it gets dark in winter. If you are on top of the world in winter you never feel the sun. It’s still dark, âBaldassare said.
At the time of the summer solstice in June, Spokane will only have a few seconds before 4 p.m. of daylight.
The tilt of the Earth is the reason it gets dark so early in winter and the reason we have different seasons. But I was curious as to why there is a tilt at all.
âWe believe the reason for the tilt is that when our solar system was still forming and things were quite chaotic, something collided with Earth causing it to fall off its axis up and down perfect for make it a little tilted, âshe said.
If you find yourself indoors for most of the winter season, you can try out a new game, curl up with a book, listen to your favorite music, do some stretching, or keep a big list of all your science questions.
What types of activities does your family enjoy doing on winter days? Talk about it with your family or friends. Then share your response and ideas in an email to [email protected] We could just submit your answers on askdruniverse.wsu.edu.
Ask Dr. Universe is a project of Washington State University.