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  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Neptune – meet Jupiter! All the angles on an AM event

September update: Neptune continues a relative close relationship with Jupiter in September 2009 and will be getting closer again later this year. To see Neptune’s position  in September 2009  go here and scroll down to the section on “Finding Neptune.”

Jupiter passes so close to Neptune this month (April 2009) you’ll be able to see both planets at the same time in binoculars or a small telescope – assuming you don’t mind getting up before the Sun 😉

OK, maybe you do mind getting up before the Sun – so don’t bother with this one. There will be two more opportunities this year, once in July and once in December. (Yes, planetary conjunctions tend to come in threes.  My own rationale for trying to catch this in May is based on the fact that these are normal hours for  me and even if they weren’t, our weather is so fickle I don’t pass up good opportunities – who knows what July and/or December will bring!)

The date to aim for is May 27, but a week either side of that will  work for either binocular or telescope view. Why bother? Because it’s events like these that give you a handle on size and distance of various objects and Neptune just isn’t that easy to find onits own, so this is a good way to go about finding it. But let’s look at the particulars. Here are a couple of charts made from Starry Nights software. The first shows you a naked eye view of the sky on May 27, 2009 at 4 am EDT  from about 42 degrees north latitude. The lesson here is simple, look low in the southeast – you may spot brilliant Venus very close to the horizon in the east and you certainly should see Jupiter, roughly a quarter way up the sky and just south of southeast. Nothing else will be nearly as bright as these two, though Venus will have Jupiter beat by about two magnitudes.  (The first magnitude  “star” near Venus will be Mars.)

naked eye view of Jupiter, Venus and Mars. Though Neptune is there, it's too faint to be seenw itht he naked eye.

Naked eye view of Jupiter, Venus and Mars. Though Neptune is there, it's too faint to be seen with the naked eye, though it should make n easy binocular target. (Click image for large version.)

This second view (below) shows what you’ll see in binoculars on May 27. A few days before that Jupiter will be even closer to the 5th magnitude star Mu Capricorni which is just a bit brighter than Jupiter’s brightest moons. Each day after May 27 it will be moving a tad to the east -left.  Neptune, at 8th magnitude, will be significantly fainter and appear  like a star in binoculars. It should be easier to see than Jupiter’s moons simply because it’s not so close as to be caught in the glare of the giant planet.  But it will certainly be easier to see in a low-power, wide-field telescope view. (Aim for a field of view of close to one degree to capture both Jupiter and Neptune nicely. )  Neptune should have a bluish hue and will show a tiny disc of about 2.3 seconds of arc in diameter.

Jupiter and Neptune as seen in binoculars on May 27, 2009, low in  the southeast about 4 am.

Jupiter and Neptune as seen in binoculars on May 27, 2009, low in the southeast about 4 am.(Click to enlarge.)

So what can you expect to see in the telescope? Not much. Jupiter’s disc is 41 seconds in diameter at this point. It’s largest moon, Gannymeade, shows a disc of about 1.5 seconds and Neptune’s disc is just a tad larger than that at 2.3 seconds. Why so small? After all, isn’t Neptune one of the “gas giants.”

Yes. But first off, it’s is significanty smaller than Jupiter – roughly one third the size. (It’s still about four times the diameter of Earth, however. For a neat guide to the sizes ofthe planets, see this Web site.)

Jupiter is about three time the diameter of Neptune and both are far, far larger than the Earth.

Jupiter is about three times the diameter of Neptune and both are far, far larger than the Earth.

But look at the distance. Jupiter is at 5.2 astronomical units  from Earth this month – Neptune is at  more than 30 astronomical units – almost six times as far away.   (An astronomical unit is about 93 million miles, the distance of the Earth from the Sun.) Does that make sense, then? Do the math! it’s simple.

Move Neptune as close to us as Jupiter and it would appear to be roughly six times larger. Six times 2.3 is 13.8. We said Jupiter was about three times bigger than Neptune. Three times 13.8 is 41.4 – and Jupiter shows in our sky a disc of 41.6 seconds at the end of the month.  Don’t you like those numbers? All approximations, but they come out very close and thus make sense.

So, will you really see the disc of Neptune? Yes – but after getting your wide angle view of both planets at once, pump up the power to get a better view of Neptune. It won’t be great – but hey, remember – you are seeing it by reflected sunlight that has taken over four hours to reach Neptune and another fours hours to bounce back here to you.

And what about Mu Capticorni? Well, it’s a nice little power house as well. As you look at it, recall that it is about 90 light years away, yet it appears about as bright as our Sun would appear  if our Sun were just 30 light years away! That means that like so many of the naked-eye stars we see, Mu is significantly brighter than our Sun.  All of that is neough to get me out of bed at 3:30 am and trundle on out to catch the sight.  Besides – I love mornings – especially those last couple hours of darkness – so peaceful, so quiet, so rich in contemplative ambiance.

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