Jupiter-Earth- by John Hlynialuk

If you are in the habit of scanning the sky (while exercising the pet pooch, perhaps) you have probably seen that brilliant "star" in the West after the Sun goes down. That object is not a mystery to the gentle readers of this column (the last two columns anyway). You know that you are looking at no star, but a planet, Venus, the brightest planet in the sky right now. So if you have been wishing on the "first star I see tonight", Venus will not work, -planets don't grant wishes. And as it turns out, there is another "fake star" up there that can also lead you astray in the wishes category.

From my location on the south edge of town, part of my view to the East is blocked by a formidable escarpment, and anything that rises on the eastern horizon does not clear the cliff for an hour or so. But there is a second Evening Star i.e.. Jupiter, in the eastern sky now and once it clears the cliff, it often appears through the ancient cedars on the top of the rocks. Looking like a gigantic firefly, it often grabs my attention.

Jupiter right now is visible all night long, rising in the East at sunset and in dark sky until sunrise. Twelve hours after rising, it sets below the western horizon as the Sun appears in the morning sky. This arrangement is referred to as an "opposition", -an astronomical term meaning that the planet is on the opposite side of Earth from the Sun. It comes to us from a time when everything was referenced to the Earth, so the term can be a bit confusing. it may help to think of the Sun at the hole of an LP record (remember those?) with Earth on an inside track and Jupiter on an outside track. At opposition all three are in a straight line: Sun-Earth-Jupiter with both planets on the same side of the Sun, and as close to each other as they can get. Furthermore, since all planets are in orbital motion (moving faster on inner orbits and slower on outer ones) the Earth is presently overtaking Jupiter in its more distant orbit and passing it.

In addition, sunlight will be hitting both the Earth and Jupiter on the same side. From our perch looking down on the planets, for this opposition, the sunlit side is on the right side of each globe. (See diagram.) So when it is nighttime on Earth, earthlings will be looking away from the Sun and what is in the sky in that direction? -the planet Jupiter, with its face fully illuminated. Thus from Earth, we see Jupiter in a dark sky totally lit up and also closest to us since we are both on the same side of the Sun. There is no better time in the entire year to see a planet like Jupiter than at opposition.

Incidentally, it is possible to see Jupiter in the daytime (Venus, too). I am sure you have noticed the Moon in the daytime sky, and if you know exactly where to look with a telescope, Jupiter can be seen as well. It can only be seen with optical aid, and even a small telescope with a motorized mount that can track objects electronically will do the trick. (Spoiler alert: Jupiter is much, much better seen at night than in the daytime.)

Although the exact instant of opposition for Jupiter was at 9 pm EDT May 8, Jupiter watching will be good for several months hence, and both Venus and Jupiter will be the first "stars" that pop out in the darkening sky. And after Venus sets in the West, Jupiter remains bright in the sky all night long. And, hold on to your touques*, the planetary extravaganza is just beginning. There are two other equally interesting planets that reach opposition this spring and summer, Saturn on June 27 and Mars on July 27. I'll have more to say about those two in future columns.

BAS has an evening viewing session devoted to Jupiter watching on Saturday May 19 and you are welcome to join us at the Fox Observatory at the outdoor ed centre near Wiarton. If you are willing to stay until 1 am or so, (hot chocolate supplied) you can get your first glimpse of Saturn as well. Our website has maps and more details.

Clear skies and happy planet-watching!

*toques are the preferred headgear for all amateur astronomers.




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