Newsletter Archive

The Solar System and the Solstice

Dec 19, 2022


It's time to celebrate the Winter Solstice!

As you may know, the December Solstice (i.e. the Winter Solstice in the Northern Hemisphere) will occur this Wednesday, the 21st. On that day, the sun will reverse its annual southward trend, and start heading northward again. The days will stop growing shorter and start growing longer (or vice versa, if you live in the Southern Hemisphere). If you have kids, perhaps you can watch the sunrise and sunset from your back yard, mark their position on the horizon with arrows of some kind, and notice that the arrows do not point in opposite directions. The sunrise and sunset do not happen on opposite sides of the horizon. The sunrise and sunset currently occur well to the southern side of true east and true west. You may also find it interesting to notice how high (or rather, low) the sun is in the sky at noon, and how long shadows are at noon.

(For a fancy project, you could press a stick or pencil through the middle of a paper plate, and follow the progression of the shadow throughout the day, marking at intervals with a pen or marker. If you are interested, I tried this with a clock face and a dowel, made a time-lapse video of the shadow, and published the results on youTube here. There are also summer and equinox versions, as well as an annual composite. By the way, if you are worried about your schedule, you don't have to do any sun-related solstice activities on the exact day of the solstice. The sun swings back and forth between solstices like a child on a swing set, passing quickly through the middle and spending most of its time turning around at the extremes. The solstice marks the exact time of standstill or stationarity of the sun — hence the name — but nobody other than mathematicians would notice any difference in the sun's behavior for several weeks on either side of that day.)

Besides marking the solstice, Wednesday might also be a good day to start a series of evening sky-watching walks. Two planets will be hanging low over the sunset, and three more will be strewn across the southern sky. All five of the visible planets will be lined up in the same sky, providing another good opportunity to visualize the “Solar System in the Sky.” If you face south just after sunset (or north, if you live in the Southern Hemisphere), you should see them lined up in a common arc, stretching across the southern sky from the sunset in the southwest to the opposite horizon in the northeast. (The arc will begin and end at the same points on the horizon no matter where you live, but it will be inclined at different angles depending on your latitude. For Southern Hemisphere viewers, it will stretch across the northern sky instead of the southern sky.)

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Facing South, Shortly After Sunset

Just over the sunset are the two planets trapped close to the sun, in space and in the sky: Mercury and Venus. They will both be difficult to spot, because they are so close to sun. Mercury will be higher, but it will also be much dimmer. However, if you have a clear view of the horizon, you or your kids may enjoy hunting for them. The ”greatest elongation“ of Mercury will be on the 21st (on the solstice coincidentally), meaning that it will be the farthest from the sun on that day. Afterwards, it will rapidly sink into the sunset, and it will be gone from the evening sky before the New Year begins. Venus, however, will slowly creep higher and higher over the sunset for the next several months, eventually becoming the glorious ”evening star“ in the late spring and early summer of next year. Jupiter and Mars should be fairly easy to spot, and Saturn shouldn't be difficult, either, once the glow of the sunset fades. Once you have found as many planets as you can, use the glow on the horizon to imagine the sun below the horizon, and then connect them all in your imagination. They should form an arc across the sky, or rather a portion of a circle surrounding you. That circle is your view of the solar system, seen from the inside.

The only thing missing from this view of the solar system is the moon, but the moon will soon join the scene as well, and help to trace the circular path by its day-to-day progression. The New Moon happens as the moon passes alongside the sun in its journey across our skies, and this month's New Moon will occur this Friday, the 23rd. Start looking for the beautiful sliver of a thin crescent moon to rise into the sunsets on Sunday or Monday (or Saturday if you're ambitious). After passing Venus and Mercury, the moon will continue its monthly journey around the circle, passing all of the other planets in turn, and eventually reaching opposition, rising in the east as a Full Moon just as the sun sets in the west.

Every month, the moon completes one lap around this circle, meaning that it passes everything else along this ”highway“ in the sky once a month. Usually, when the moon passes a planet (or the sun) in the sky it passes alongside, like a car passing another on the highway. But as you may remember, when the moon reached Mars last month it actually eclipsed or ”occulted“ Mars, at least for viewers in the Northern Hemisphere. (I hope you were able to enjoy this event as much as I did. This was actually the first ”occultation“ that I've seen with my own eyes, and I thought it was quite a sight, especially through binoculars!) Well, whenever the moon occults a star (or ”wandering star“, i.e. planet), it tends to repeat the performance for several months in a row before its track shifts and it starts flying wide of the star again. And so the moon will ”strike“ Mars again this time around. On January 3rd, there will be another lunar occultation of Mars, although this time it will only be visible from Madagascar and southern Africa. For everybody else, the moon will pass very close to Mars, but never touch it in the sky. (And yes, it will happen again the next time around. On January 30, there will be yet another occultation of Mars, this one visible from Central America and the southern United States.) I suggest keeping your eyes on the Moon as it passes Mars, even if it won't eclipse Mars from your perspective. Just as the lineup of planets is your chance to visualize the Solar System in the sky, so the passage of the moon very close to a star or planet is your chance to see with your own eyes motion in outer space. You can see for yourself the motion of the moon in its orbit over the course of an hour or three, using a bright star or planet as a reference point.

Incidentally, you may notice that Jupiter is high in the south around the time of sunset. If you point to the sun with one arm, and point to Jupiter with the other arm, you may notice that your arms form very nearly a right angle. Jupiter is currently very near to ”quadrature“, which means that it is a quarter-turn from the sun along the circle, halfway between ”conjunction“ and ”opposition". The exact date of the quadrature of Jupiter is one day after the solstice, and one day before the New Moon, on Thursday the 22nd.

Happy Viewing!

John