What to Know About Winter Solstice & the ‘Great Conjunction’

NASA
Editor's note: Dr. William Teets is the director of the Dyer Observatory at Vanderbilt University. In this interview he explains what will and will not happen during the winter solstice on December 21st. On the same day, another cosmic phenomenon called "the great conjunction," where Saturn and Jupiter can both be seen with the naked eye, appear extremely close to each other.
What happens at the winter solstice?
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This year's winter solstice will take place on December 21st. Then the sun appears lowest in the sky in the northern hemisphere and is at its southernmost point above the earth - directly above the Tropic of Capricorn. For people who live at 23.5 degrees south latitude, this day not only marks their summer solstice, but they also see the sun directly above them at local noon. Then the sun creeps north again.
The following sequence of images shows the path of the sun through the sky at different times of the year. You can see the sun being highest in the northern hemisphere sky in June, lowest in December, and halfway between these positions in March and September during the equinox.
Winter solstice is the shortest day in the northern hemisphere, but not the day with the latest sunrise and the earliest sunset. How is that possible?
The winter solstice does not coincide with the last sunrise or the earliest sunset. These actually occur about two weeks before and two weeks after the winter solstice. This is because we are changing our distance from the Sun due to our elliptical, non-circular orbit, which changes the speed at which we are orbiting.
If you look at where the sun is at exactly the same time of day on different days of the year, you will find that it is not always in the same place. Yes, the sun is higher in summer and lower in winter, but it also moves back and forth from the average noon position which also plays a role when the sun rises and sets.
It should also be remembered that the seasons are due to the axial tilt of the earth, not our distance from the sun. Believe it or not, we are closest to the sun in January.
What is the great conjunction?
Saturn and Jupiter have appeared pretty close together in the sky all year round. But on December 21st, Saturn and Jupiter will appear so close together that some people will find it difficult to see them as two objects.
If you have binoculars, you can easily see both planets. Even in a small telescope, you can see both planets in the same field of view at the same time, which is really unknown. That is what makes this conjunction so rare. Jupiter and Saturn seem to meet roughly every 20 years. Most of the time, however, they are not nearly as close together as we will see them on Monday, December 21st.
For comparison, there was a great conjunction back in 2000, but the two planets were separated by about two full moon widths. This year the orbits will take them to where they appear to be about a fifth of the diameter of a full moon.
We encouraged people to only look at these planets through their eyes until December 21st. You will actually be able to see how much you seem to be moving in a single day.
The next time they come together this close to our heaven it won't be another 60 years, so this will be a once in a lifetime event for many people. In fact, the last time they got this close was in 1623, but it was really difficult, if not impossible, to see them then because they appeared much closer to the sun and set soon after. Go back 400 years to 1226 and this would have been the last time we had a good look at this type of conjunction.
What advice would you give to people who want to see the great connection?
If the weather permits at the Dyer Observatory, we'll stream a live view of the conjunction from one of the observatory's telescopes and I'll be available to answer any questions. Even if you don't have a telescope or binoculars, be sure to go out and check this very rare alignment with your own eyes. Remember, they set just after sunset. So be ready to watch them right at dusk!
William Teets is the associate director and astronomer at Vanderbilt University's Dyer Observatory
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