Why was it easier to see Jupiter 2000 years ago?

Question

There's a GCSE-style question talking about some Chinese astronomer seeing Ganymede without a telescope in my science revision guide thingy. The answer to the question isn't even hinted beforehand in the 'fact bank', and I've become interested in what the answer could be...

Answer 1

Gan De is credited with having seen one of the Jovian moons with the naked eye in 364 BC. It is assumed to be Ganymede as it is the brightest of the four.

Gan De (4th century BC) was a Chinese astronomer/astrologer born in the State of Qi also known as the Lord Gan (Gan Gong). Along with Shi Shen, he is the first in history to compile a star catalogue, followed by the Greek Hipparchus who was the first in the Western tradition to create a star catalogue.

He wrote two books, the Treatise on Jupiter and the 8-volumes Treatise on Astronomical Astrology, both of which have been lost.

In 1973, a similar catalogue by him and Shi Shen was uncovered in Mawangdui and was arranged under the name of Divination of Five Planets, it records the motion of Jupiter, Saturn, Venus and other planets in their orbits between 246 BC and 177 BC.

He is reported to have seen one of the moons of Jupiter (possibly Ganymede) with his naked eye in 364 BC, long before Galileo Galilei's celebrated discovery of the same in 1610 (all four of the brightest moons are technically visible to the unaided eye, but in practice are normally hidden by the glare of Jupiter).

By occluding Jupiter itself behind a high tree limb perpendicular to the satellites' orbital plane to prevent the planet's glare from obscuring them, one or more of the Galilean moons can be spotted in favorable conditions.

Perhaps Gan De practised some such masking out of Jupiter as a technique. Perhaps he had exceptionally good eyesight. Certainly China in the 4th Century BC was not industrialised for air pollution to interfere with astronomers' acuity of vision and there was no electricity to cause light pollution.

Answer 2

Ganymede is theoretically visible to the naked eye. It is simply that Jupiter's brightness may flood it out.

OK, there is a lot of light pollution now, but there are still areas of the world where the skies are crystal clear. The 4 largest moons of Jupiter are easily visible with binoculars, so I would think that from some remote, high location, it would be possible to see Ganymede with the naked eye when it is at its widest elongation from Jupiter.

PS - of interest is that in Gullivers Travels, the Laputians describe the moons of Mars. This was written in 1726, but Mars two moons were not discovered until 1877. In fact, in 1726 telescopes were not powerful enough to see the two very tiny moons. Yet, Jonathan Swift describes the orbits and sizes of the moons fairly accurately in his book, 150 years before the moons were discovered.

Food for thought!!!

Answer 3

The sky would have definitely been clearer and darker 2000 years ago. Also because you would not have artificial lights it would get darker sooner, giving you more observing time.

Now Ganymede has a 4.6 magnitude. On the BEST conditions you can see stars of magnitude 6. So while theoretically possible the fact remains that in order to see it the naked eye you would have to be able to distinguish it form Jupiter, that has a -2.8 magnitude that is 912 times brighter. So put a LED right beside a car's headlight go back 50 yards and tell the apart. Not likely even in the darkest of the nights.

Answer 4

all the pollution in the air. And all the light pollution. Like now, unless you go to somewhere really clear like northern canada, or the middle of the desert, it's difficult to see more than maybe a few hundrend stars, where before people could see thousands. Now we only see the brightest ones, that can get through all that air and light pollution.

Answer 5

2000 years ago there were no electric lights and the atmospere was clean. Now there's too much crap in rhe air.

Answer 6

2 things: light pollution and air pollution.

All the lights we have in out cities are making the night not as dark as 2000 years ago, and all the garbage in the atmosphere is making the air less transparent.

Answer 7

interesting answers, it makes you wander why 1600 years later they were arguing that the earth was round and the Earth was the centre of the universe

Answer 8

the air would have been cleaner and darker than it is nowadays, he also might have just had really good eyesight

Answer 9

No air pollution, Dummy.

No "sky-glow" from city and street lighting.

Answer 10

because the earth was created almost at that time. and also jupiter was like a sun because when the sun was made somethings were in space and created the sun then the things that were left created jupider.