 year, you may have already noticed that sunset is a few minutes later now than at the beginning of the month. In June, the North Pole was tilted towards the sun as much as possible. Since then, the North Pole has tilted a little more away from the sun each day. Days have been getting shorter because each day the sun has taken a slightly lower path across the sky. Sunrises have been getting earlier and sunsets have been getting later. By late November the sun had already gotten about as low as it is now. As the day to day difference in the sun’s height gets smaller, another effect begins to dominate.
Earth’s orbit is not a circle; it is an ellipse. The orbit is almost a circle, however; the eccentricity (out-of-roundness) is just 0.016, where 0 is a perfect circle and 1 a parabola. This is enough of a difference to bring Earth slightly closer to the sun in early January and take it slightly farther away in early July. Therefore, Earth is now beginning to make its closest approach to the sun (called perihelion). As a result, Earth is speeding up on its orbit. This causes sunrise, local noon, and sunset to occur just a little later each day. By the 21, sunset will occur at 5:27 pm, as opposed to 5:22 pm on Dec. 2 (the actual date of the earliest sunset). Sunrise, however, will have shifted from 7:00 am to 7:13 am. Thus, that days are still getting shorter even though the sunsets are a little later.
Many people assume that the winter solstice should be the coldest day, but this is usually not true. January is usually colder. Although days get a little bit longer and the sun a little bit higher beginning Monday, it takes quite awhile for this to add up to an appreciable difference in the Sun’s height in the sky and in the amount of light and heat reaching the arctic. Frigid air masses continue to form in the arctic and move across the Northern Hemisphere throughout January, February, and often March. Although the sun is higher in those months than in December, the air can be just as cold if not colder.
Hopefully, we are getting all of our cloudy, gloomy weather over with , and the solstice will be sunnier. If so, you can join us on the museum sundial at noon on Monday, Dec. 21 to observe the sun! This is one of the Fun Hundred events celebrating our 100 anniversary here at the Museum. On top of the gnomon on our sundial is a silver ball with three sets of holes, which allows the sun to shine through pairs of lenses near each solstice or equinox. To account for cloudy weather, our gnomon’s holes are big enough that the sun aligns with them for a few days before and after the exact equinox or solstice date. The holes aligned with the winter solstice are so big that you can still project the sun’s image through them deep into January! If the weather does not cooperate Monday, you can come and observe the sun on our sundial near noon on any day in the next few weeks.