Newsletter Archive

The Dance of the Wanderers

January 18, 2023


Hello Everyone!

Coming up in about a week and a half, we have the last of a monthly series of Moon-Mars collisions. (As you may recall, these are more formally known as "lunar occultations of Mars.") In the meantime, we have a lovely evening scene full of "landmarks" to look for. Also, if you want to go Mercury-hunting, that elusive planet will emerge from hiding and hang out over the sunrise for the next few weeks before sinking back into the sunrise. (The "greatest elongation" of Mercury, meaning the greatest distance from the Sun in the sky, will occur on the 29th.)

The Evening Scene

Of the five visible planets, all but Mercury are currently arrayed in the evening skies for your viewing enjoyment. If you live in the northern mid-latitudes and you go outdoors about an hour after sunset and face south, you should see something like the following picture. (If you live in the Southern Hemisphere, you can see more or less the same scene, but you need to face north and hold this picture upside-down. If you live near the Equator, you'll need to tilt back and hold this picture over your head. Also, you should be able to view, save, or print a larger version of this image by right-clicking.)

The Southern Sky, About an Hour After Sunset

Regardless of where you live, you should see the same arc of planets stretching across the sky. Reading from east to west, you should find Mars, Jupiter, Saturn, and finally Venus low over the sunset. I hope I'm not becoming monotonous, but this is a point worth repeating: If you pay attention to the planets in the sky, you find that they wander around, but they always wander within the same "ring" or circular "highway" in the sky. It is as if they are all moving in the same plane in space, and this circle surrounding us is what the plane of motion looks like from our point of view, from inside the plane. As we might say in the modern world, this "planet highway" that we see in the sky is our view of the "Solar System" in the space around us. And whenever you see several planets together in your sky, this is your chance to connect the dots in your imagination, and to visualize the solar system yourself. For the next month or so, the evening sky will not only give you the potential for a beautiful evening walk, but also an opportunity to see the Solar System in the space around you with your own eyes.

If you keep paying attention day-by-day and week-by-week, you may also notice the motions of the wanderers as they move around in space. You may remember that Mars passed opposition (meaning the opposite side of the sky from the sun) about a month and a half ago. At opposition, Mars rose in the east just as the sun set in the west, but it has been creeping slowly westward ever since, so it is now high in the southeastern sky at sunset. Jupiter and Saturn have been slowly creeping in the same direction, towards the sunset. Venus is currently wandering along the highway in the opposite direction. It was the "Morning Star" for much of last year, but then it sank into the sunrise, passed the Sun, and emerged anew over the sunset around Halloween of last year. Since then, it has been very gradually climbing and brightening over the sunset, and it will become the glorious "Evening Star" before long. It is already quite bright, and will continue to climb and to brighten as the weeks go by. It will pass Saturn going in the opposite direction in a few days, on the 22nd, giving us a good chance to see the brightest planet and the slowest planet (or more precisely the slowest naked-eye planet) very close together in the sky.

The New Moon this month will occur this Saturday the 21st, and a day or two after that will be a good time to start looking for the beautiful sliver of a crescent moon low over the sunset. The 23rd, one day after the conjunction of Venus and Saturn and two days after the New Moon, should make a particularly beautiful sunset scene, with the crescent Moon, Saturn, and brilliant Venus all together over the fading glow of the Sun. During the following week, follow the Moon as it travels down the highway from west to east, waxing and passing each planet in turn as it heads towards opposition.

As the skies darken after sunset, you can also enjoy some of the bright constellations decorating the evening skies. In particular, notice the characteristic V-shaped cluster of red stars forming the horns of Taurus, and another tight grouping of lovely blue stars nearby, also technically in Taurus. These two star clusters are the “Hyades” and the “Pleiades”, and they are gorgeous in binoculars. You may also notice that they bracket the "planet highway", with one star cluster on each side. This pair of star clusters is sometimes known as the "Golden Gate of the Ecliptic." (The "ecliptic" is the exact path taken by the Sun through the zodiac, a little like the "centerline" of the “planet highway.”) Do you suppose that if you watch the Moon as it passes along the highway, you will see it pass through the Golden Gate in a week or two?

The Lunar Occultation of Mars

Every month, the Moon passes the Sun and all of the planets as it completes its monthly lap around the circular highway. You may recall that for the last couple of cycles, the Moon has passed so close to Mars in the sky that (for some locations on Earth) it actually appeared to pass in front of Mars, blocking it from view for several minutes to an hour. In formal terms, there was a "lunar occultation of Mars." This month's lunar occultation of Mars will be the last in the current series and also the last for several years. (Actually, there will be one more at the end of February, but you'll have to live at the North Pole to see it.) It will be visible in the southern United States, Mexico, Central America, and the Caribbean, and it will occur in the late evening or around midnight of the 30th, depending on your location. Elsewhere in the Western Hemisphere, the moon will never "touch" Mars in the sky, but it will still pass very close. However, even if you can't see the occultation, if you live in the Americas, this is still a good opportunity to see motion in outer space with your own eyes. By comparing the moon to a nearby "landmark" for an hour or two, you can can see the moon move for yourself.

There are several problems with trying to observe lunar occultation, and one of them is that the Moon is sometimes so bright that it makes it difficult to see nearby stars. This is one reason to take advantage of an occultation of a planet when they occur, because planets are usually brighter than "true" stars. If the Moon is not full, this is also a big help. This month's lunar occultation of Mars will occur with a nearly first-quarter moon, so the Moon won't be quite as overpoweringly bright as it was last month. This month's Moon-Mars passage should be quite special and impressive, and if you live in the southwestern United States it even occurs in the convenient evening hours!

So if you live in the Americas, pay attention to the Moon as it descends in the western sky in the late evening of the 30th. In general, the farther to the west you live, the earlier in the evening the passage will occur, and the higher in the sky the Moon will be. I'll give details for a few specific cities below, and if you'd like to see a map or search for timing details for more cities, you can try the lunar-occultations website.

San Diego —The occultation will occur from roughly 6:30-7:30 PM, while the moon is still very high in the western sky.
Mexico City —The occultation will occur from roughly 11 PM on the 30th to 12:30 AM on the 31st, with the Moon roughly halfway up in the western sky.
Miami —The occultation will occur from roughly 12:30-1:30 AM in the morning of the 31st, with the Moon fairly low over the western horizon.

Note that Mexico City will see Mars pass behind the center of the Moon, so the occultation will last longer there than elsewhere. San Diego and Miami lie farther to the north, so they will see Mars pass behind the Moon a bit above center, and the occultation won't last quite as long there. If you live farther north than San Diego or Miami, you will probably see the Moon miss Mars, but it will still pass very close underneath it. If you live near the West Coast, look for the passage to occur at about the same time as the occultation in San Diego, and if you live near the East Coast, look for the passage to occur around the same time as the occultation in Miami.

If you don't like the map at the lunar-occultations website I linked to previously, how about this one? It's a little less technical, and larger and prettier. If you live in the Western Hemisphere, you may find it interesting to examine the boundaries of the region of visibility on the map. If you live near Columbia or the Dominican Republic, you will see the occultation begin while the moon is very low over the western horizon, and then the moon will set before Mars re-emerges from behind the Moon. If you live in the southern United States, please pay special attention to the upper boundary line on the map. If you live near that line, you may get to see a "grazing occultation," in which Mars will appear to graze the edge (or "limb") of the moon in the sky. If you watch the passage through binoculars or a telescope, you may be able to detect motion in real time. And if you are very lucky, even without magnification, you may be able to see Mars wink on and off as it passes behind mountains and valleys on the moon!

After this occultation of Mars, there will be no more for a while, although the moon will continue to pass quite near to Mars for a few more months. There will also be an occultation of Venus in late March, visible in portions of China and Indochina. (That one will occur in the skies over India and southern Africa as well, but unfortunately during the daytime. Although sometimes Venus is so bright that you can even see it in the daytime...)

Unfortunately I'm not expecting to enjoy much of the sky myself for a while, as here in Iowa we are in the midst of ongoing gray winter gloom. Here's hoping the skies are clearer where you live!

Until next time!

John