In the Northern Hemisphere, December is known for having the shortest amount of daylight and the longest nights. This happens because the Earth’s axis is tilted, pointing the Northern Hemisphere away from the Sun. But here’s something surprising: December actually has the longest days of the year! How can that be?
First, let’s talk about what a “solar day” is. A solar day is the time it takes for the Earth to rotate so that the Sun appears in the same position in the sky as it did the day before. This is slightly more than 24 hours because the Earth is not only spinning on its axis but also moving around the Sun. So, it has to rotate a bit more than 360 degrees for the Sun to be in the same spot again.
If the Earth’s orbit were a perfect circle and its axis wasn’t tilted, each day would be the same length. However, the Earth’s orbit is elliptical, meaning it’s sometimes closer to the Sun. When the Earth is closer, it moves faster due to gravity, which means it travels a longer distance in its orbit in 24 hours. This requires the Earth to rotate a little more, about 0.033 degrees extra, making the day slightly longer by about eight seconds.
Additionally, the Earth’s axis is tilted, which is why we have seasons. This tilt means that at certain times of the year, like December, the Earth has to rotate a bit more, about 0.088 degrees, to bring the Sun back to the same spot in the sky. This adds about 21 extra seconds to the day.
Interestingly, we live in a time when the Earth’s closest approach to the Sun happens around the same time as one of the two times of the year when the Earth’s tilt is directed towards the Sun. These two factors combine to make December 22nd the longest solar day of the year, lasting 86,430 seconds, which is 30 seconds longer than average.
You might wonder what happens to those extra 30 seconds. They don’t just disappear! Instead, they shift into the next day, December 23rd, and then into December 24th, and so on. This is why solar noon, the time when the Sun is directly overhead, happens about 30 seconds later each day around the solstice.
This shift also explains why, in the Northern Hemisphere, the earliest sunset occurs a few weeks before the solstice, and the latest sunrise happens a few weeks after. So, while December might feel like it has the shortest days, in terms of sunlight, it’s actually when the days are longest if you measure them by the Sun!
Use a globe and a flashlight to model the Earth’s rotation and orbit around the Sun. Observe how the tilt of the Earth’s axis affects the length of a solar day. Record your observations and discuss how the Earth’s position in its orbit influences day length.
Work in pairs to calculate the additional rotation needed for the Earth to complete a solar day in December. Use the information about the Earth’s orbit and tilt to determine how these factors contribute to the extra seconds added to each day.
Over a week, track the time of solar noon using a sundial or an online solar calculator. Record the time each day and analyze how it shifts. Discuss why solar noon occurs later each day around the solstice.
Research the concept of elliptical orbits and how they differ from circular orbits. Create a diagram to illustrate the Earth’s elliptical orbit and explain how this shape affects the speed of the Earth’s travel and the length of a solar day.
Participate in a debate about the effects of the Earth’s tilt and orbit on seasonal changes. Use evidence from the article to support your arguments about why December has the longest solar days despite having the shortest daylight hours.
December – The twelfth month of the year, often associated with the winter solstice in the Northern Hemisphere, when the Earth’s axial tilt is farthest from the Sun. – In December, the Northern Hemisphere experiences the shortest day and longest night of the year due to the Earth’s tilt.
Solar – Relating to or determined by the Sun. – Solar energy is harnessed from the Sun’s rays and can be used to generate electricity.
Earth – The third planet from the Sun in our solar system, which supports life and has a unique atmosphere and surface conditions. – Earth rotates on its axis, causing the cycle of day and night.
Axis – An imaginary line about which a body rotates. – The Earth spins around its axis, which is tilted at an angle of about 23.5 degrees.
Tilt – The angle at which an object is inclined from its vertical position. – The tilt of the Earth’s axis is responsible for the changing seasons.
Orbit – The curved path of a celestial object or spacecraft around a star, planet, or moon. – The Earth completes one orbit around the Sun every 365.25 days.
Sunlight – The light and energy that come from the Sun. – Sunlight is essential for photosynthesis, which allows plants to produce food.
Gravity – The force that attracts a body toward the center of the Earth, or toward any other physical body having mass. – Gravity keeps the planets in orbit around the Sun.
Hemisphere – Half of the Earth, usually divided into Northern and Southern by the equator, or Eastern and Western by the prime meridian. – The Northern Hemisphere experiences summer while the Southern Hemisphere experiences winter.
Seconds – A unit of time, one-sixtieth of a minute. – It takes approximately eight minutes and twenty seconds for sunlight to travel from the Sun to the Earth.
