The Solar System at a Glance

Welcome to Summer!

I don't know about where you live, but summer has definitely arrived here in the Midwest. It's hot, and humid, and sometimes it stays that way overnight. Yuck. Time to dust off the air conditioners.

And we haven't even reached the solstice yet. The June Solstice (i.e. the Summer Solstice in the Northern Hemisphere) will arrive in a little under a week, next Tuesday the 21^st. That will be the longest and most irradiated day of the year, and the official beginning of summer. (You may remember that we use the solstices to mark the beginnings of their respective seasons, rather than the centers, because the weather tends to lag a bit behind the solar irradiation.) In the Southern Hemisphere, of course, it will be the darkest day of the year rather than the sunniest, heralding the arrival of winter, and if you live near the Equator … well, it will be a day like any other.

This month, I want to draw your attention to a special performance taking place in the eastern pre-dawn skies. Over the sunrises this month, something remarkable is happening.

The Scene

The planets never appear to us in the sky except along a narrow “highway” through the constellations, running around us in a circle. This circular ring is our view of the solar system, seen from the inside, and we can visualize it in the sky whenever we can find two or more planets to mark the path. (We can also use the zodiac constellations as markers — that's what the zodiac was invented for — but those constellations are much broader than the “highway”, and half of them are dim and difficult to find.)

Every once in a while, we are lucky enough to see all five visible planets appear together in the sky at the same time. And on rare occasions, all seven planets appear together in a lineup — the five “naked-eye” planets plus the two “telescopic planets” Uranus and Neptune. (We don't count the Earth, because we're standing on it, and we don't count Pluto, because it has been demoted, and because you can never see it anyway, even in a powerful telescope.)

This seven-planet lineup is our treat in the morning skies this month. All seven planets are lined up above the sunrise, and even the moon will join the scene later this month, giving us the entire solar system arrayed along the zodiac over the sunrises. From our point of view, the solar system occupies a ring surrounding us, and for the month of June, the most interesting bodies in the solar system will all be clustered on one side of the ring, within an arc of 100° or 110° or so. The sun, the moon, and all of the planets will be concentrated into a little more than a quarter of the full circle.

If you like to enjoy the impressive beauty of the planets through a telescope — the phases of Venus, the moons of Jupiter, the rings of Saturn, and the mountains on the Moon — then the pre-dawn skies of June will give you a spectacular buffet. With a powerful telescope, you may even see features in the face of Mars, or the colorful bands of Jupiter … although Uranus and Neptune will still look like dim stars.

The following picture shows the view facing east at approximately 5 am, or roughly ¾ of an hour before sunrise, on June 24^th, from a latitude of roughly 40-45° north. The farther to the north or south that you live, the more or less tilted the lineup will appear. If you live just south of the Equator, near the latitude of Rio de Janeiro, the lineup will stand nearly vertically, and deeper in the Southern Hemisphere the lineup will appear tilted to the left instead of the right.

(I apologize for the small labels. If you right-click on the image, you should be able to download it and view it enlarged with a separate image viewer.)

Above the eastern horizon, Northern Hemisphere viewers will see the Great Square of Pegasus, with another star standing above the top corner like an umpire behind one corner of a baseball diamond, and a long, graceful arc extending from another corner of the square, making it look like a “Super Duper Dipper” (as my students used to call it.) These are the brightest and most recognizable stars of the constellations Andromeda and Pegasus. The planets, the moon, and the sun will array themselves along an arc underneath these two constellations.

You may notice an interesting coincidence, or semi-coincidence: The 5 naked-eye planets are arranged in the sky in the same order as their physical distance from sun. The closest (and speediest) planet is closest to the sunset, and the farthest (and slowest) visible planet is the farthest from the sunset. If you want to try to find the two remaining planets with a telescope, Uranus is about halfway between Venus and Mars, and Neptune is about a third of the way from Jupiter to Saturn.

The Schedule

The view will be similar for much of June, except for the moon, which will zoom past the planets in the second half of the month. The moon was full two days ago, on the 14^th, so if you search the morning skies tomorrow, you will find the moon on the far side of the sky from the sunrise. For the next fortnight it will cross the sky, following the “highway” and shrinking as it goes, and it will pass the sun as a new moon on the 28^th. Between now and then, it will pass all of the planets in turn, starting with Saturn on this Saturday or Sunday morning (depending on your longitude).

The planets will move more slowly than the moon, shifting their positions gradually as the days go by. Today, the 16^th, is the “greatest elongation” of Mercury, meaning it is as far as it gets from the sun in the sky. From now on, it will sink fairly rapidly (compared to the other planets) into the sunrise, and will disappear from view in early July. Venus passed greatest elongation some time ago, and has been sinking very gradually into the sunrise for several months. It will continue to do so throughout June. The outer planets will move very slowly in the opposite direction, creeping gradually higher over the sunrise throughout June.

If you are an avid sky-watcher and an exceptionally early riser, maybe you'll want to enjoy the beauty of the sky before sunrise every day or two for the rest of the month. If you'd rather pick a few special days, I recommend concentrating especially on the 24^th-26^th. On the 24^th, for viewers in the Americas, there will be a beautiful, even spacing, with the crescent moon filling the gap between Venus and Mars. (This is the situation shown in the graphic above.) Starting around this day, the crescent will also be thin enough that you might also be able to see the lovely glow of “earthshine” on the dark side of the crescent, adding to the beauty of the scene. On the 26^th, viewers in the Americas will see the lovely pairing of a thin crescent moon and brilliant Venus, very close together.

The exact timing of the moon's passing of each planet will depend on where you live. The moon will continue to advance through the planets as dawn advances through the time zones of the world, so the moment when the moon passes closest to each planet could occur at any time of day, depending on your longitude. On the morning of any given date, more westerly longitudes will see the moon farther down the line than more easterly longitudes. For viewers in the Americas, the moon will pass Saturn on the morning of the 18^th. It will be near Jupiter on the 21^st, near Mars on the 22^nd, near Venus on the 26^th, and near Mercury on the 27^th. Viewers in Australia will see a similar timing, but about a day later. Viewers in Europe and Africa will see the moon in the intervals between the planets on most mornings. (If you would like a precision map and time-table for your exact location, I recommend the software Stellarium, which has an online version.)

A Special Guest

I have to make note of a special guest who will appear briefly in the morning scene, although not everyone will be able to see it. When I was preparing the graphic above, I noticed an extra bright dot, brighter than any star and brighter even than Venus or Jupiter, zooming down the planet lineup on the morning of the 24^th. It crossed the sky, not in days, but in minutes. This was the International Space Station, which will be visible here in central Iowa for a few minutes starting at about 4:52 am on the morning of the 24^th, passing right below the planets. Where and when the ISS will appear in your sky depends entirely on where you live — If you'd like to check for sightings in your local area, NASA has a nice website for that.

Happy Viewing!

John