Astroblog: Unaided eye Callisto Saturday April 8

Thursday, April 06, 2006

Unaided eye Callisto Saturday April 8

Orientation of jupiter and its Moons at 10:00 pm (local time anywhere in Australia).
It was a discussion with a Velikovsky supporter that alerted me to the fact that the Jovian Moons are potentially visible to the naked eye. As you may know, Velikosky claimed that Venus was originally a "comet" ejected from Jupiter. He also claimed that the myth of Minerva springing from Jupiter/Zeus's forehead was based on visual observation of Venus "erupting" from Jupiter, accompanied by bright lighting bolts from Venus to Jupiter.

Unfortunately, they would have never seen it. All of Jupiter's Moons are bright enough to see with the unaided eye (Ganymeade, the brightest is mag 4.6). Although Callisto at Magnitude 5.7 is just above the limit, and you would need dark skies to see it. Yet you can see none of them (unless you follow the instructions below). Part of this is distance, and the other part is brightness.

The human eye can generally distinguish objects 4' (arc seconds) apart (people with excellent vision can see objects 1-2' apart), although more practically this limit is more like 8-25' depending on age and how good your eyesight is. Io and Europa never get much more than 2' and 3' away from Jupiter respectively, so most people would never be able to see them. Ganymeade and Callisto get to about 5' and 10' respectively at Jupiter's closest approach, and could theoretically be seen, but the brightness of Jupiter overwhelms their light.

Venus would be quite a bit brighter than the Jovian Moons at Jupiter's distance, but still substantially less than Jupiter. A Venus sized object at the distance of Jupiter could possibly be seen at the 8' limit. This would be long after any eruption and "lightning bolts" had happened. So rather than seeing Venus dramatically erupt from Jupiter, exchanging lightning bolts with it, a keen-eyed shepherd might have seen a moderately bright star creep slowly away from Jupiter over several hours/days, provided that Jupiter was at closest approach and Venus was ejected at right-angles to the shepherd's line of sight (otherwise they may not have noticed it for days). Casual observers would never have noticed. Hardly the stuff of legend.

But there is a way to see Callisto without either telescope or binoculars. Like seeing Venus in Daylight, you need to block out the light of Jupiter. This works best when Callisto is at its furthest from Jupiter (and when Jupiter is reasonably close to Earth, so the apparent separation is largest). You will also need dark skies of course, so if you are not in the country, you might like to plan a nighttime trip to the country. The night of April 8 is the first good opportunity we have to see Callisto (there are about 6 others in the next two months, 17 and 25 April and 3 May and 11 May are probably the best).

You will need to be able to see Jupiter (here is the location map for Jupiter), and to have a large object like a wall or roof to block out Jupiter’s light. Because Jupiter is rising straight up, Jupiter's Moons are above it, so you need something that blocks Jupiter from below (like a roof line) rather from the side (until around 1.00 am, when Jupiter levels out). The image above shows how Jupiter will look in binoculars at 10:00 am. With Callisto above and to the left a bit of Jupiter, slowly move so that Jupiter is just covered by the obscuring object, and Callisto should be just visible as a faint star just above the obscuring object. You may need to check the appearance of Jupiter and it's Moons in binoculars, to be sure of what you are seeing.

It will be a challenge, but it should be an interesting challenge.

# posted by Ian Musgrave @ 10:43 pm