Big Venus and small Jupiter: Galileo’s astronomical puzzle is solved after 400 years

A visual illusion that makes Venus look bigger than Jupiter has finally been explained

Science Editor

Scientists have finally come up with an explanation for a visual illusion that was first identified in the 16th Century by Galileo Galilei, who noticed how large the planet Venus appears to the naked eye when compared to Jupiter – which is quite the reverse when seen through a telescope.

Venus is nearer to Earth than Jupiter and therefore appears brighter in the night sky, however this alone cannot account for its larger-than-life appearance. There must be another reason to do with the way the eye perceives light compared to the optical reality of a telescope, scientists said.

Viewed directly with the naked eye, Venus appears to have a “radiant crown” which makes it look eight to ten times bigger than Jupiter even though Jupiter is four times larger when seen from Earth.

Galileo was the first to realise that this radiant crown was something to do with human perception, or, as he described it, an “impediment of our eyes” which the telescope eliminated, but he put it down to some kind of optical interference to the light from the planets as the light entered the human eye.

However, scientists have now shown that the effect is caused by the way the light-sensitive cells at the back of the eye respond to images of different intensity set against a dark background. Venus appears larger because its brighter-than-Jupiter image is much exaggerated by the visual centres of the brain to create a bigger radiant crown than Jupiter’s, according to the study published in the journal Proceedings of the National Academy of Sciences.

They believe the effect influences the way we see everything because the human retina and brain are finely tuned to respond to the contrast between light objects against a dark background. This makes them appear larger than light objects of the same size set against a light background, said Jose-Manuel Alonso of the State University of New York College of Optometry.

“Galileo was the first to say that our eye was distorting reality. He could see that Venus appeared to be much larger than Jupiter when seen with the naked eye and that the opposite was true when he looked through his telescope,” Dr Alonso said.

Galileo said that the effect was some kind of size illusion created by the eyes. “Either because their light is refracted in the moisture that cover the pupil, or because it is reflected from the edges of the eyelids and these reflected rays are diffused over the pupil, or for some other reasons,” Galileo wrote.

The 19th Century German physicist Hermann von Helmholtz came nearer to the truth when he said that the “irradiation illusion”, as he called it, was caused by our sensation of the object and not by the optics of the eye.

“The latest research demonstrates that the sensation Helmholtz used to explain the irradiation illusion is a ‘non-linear’ response of the visual system when objects are presented on dark backgrounds,” Dr Alonso explained.

The edges of a light object appear blurred and this is effectively magnified by the brain so that the entire object appears bigger than it should. Venus, being nearer to Earth, is brighter than Jupiter and so it appears bigger against the dark background of the night sky, he said.

Start your day with The Independent, sign up for daily news emails
News
election 2015The 10 best quotes of the campaign
News
A caravan being used as a polling station in Ford near Salisbury, during the 2010 election
election 2015The Independent's guide to get you through polling day
News
people
Voices
David Blunkett joins the Labour candidate for Redcar Anna Turley on a campaigning visit last month
voicesWhat I learnt from my years in government, by the former Home Secretary David Blunkett
ebooks
ebooksA celebration of British elections
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Ashdown Group: Marketing Manager (B2B) - Romford - £40,000 + car

£35000 - £40000 per annum + car and benefits: Ashdown Group: Marketing Manager...

Ashdown Group: Helpdesk Analyst - Devon - £20,000

£18000 - £20000 per annum: Ashdown Group: Helpdesk Analyst - Devon - £20,000 ...

Ashdown Group: Data Scientist - London - £50,000 + bonus

£35000 - £50000 per annum + generous bonus: Ashdown Group: Business Analytics ...

Ashdown Group: IT Project Coordinator (Software Development) - Kingston

£45000 - £50000 per annum: Ashdown Group: IT Project Coordinator (Software Dev...

Day In a Page

General Election 2015: ‘We will not sit down with Nicola Sturgeon’, says Ed Balls

'We will not sit down with Nicola Sturgeon'

In an exclusive interview, Ed Balls says he won't negotiate his first Budget with SNP MPs - even if Labour need their votes to secure its passage
VE Day 70th anniversary: How ordinary Britons celebrated the end of war in Europe

How ordinary Britons celebrated VE Day

Our perception of VE Day usually involves crowds of giddy Britons casting off the shackles of war with gay abandon. The truth was more nuanced
They came in with William Caxton's printing press, but typefaces still matter in the digital age

Typefaces still matter in the digital age

A new typeface once took years to create, now thousands are available at the click of a drop-down menu. So why do most of us still rely on the old classics, asks Meg Carter?
Discovery of 'missing link' between the two main life-forms on Earth could explain evolution of animals, say scientists

'Missing link' between Earth's two life-forms found

New microbial species tells us something about our dark past, say scientists
The Pan Am Experience is a 'flight' back to the 1970s that never takes off - at least, not literally

Pan Am Experience: A 'flight' back to the 70s

Tim Walker checks in and checks out a four-hour journey with a difference
Humans aren't alone in indulging in politics - it's everywhere in the animal world

Humans aren't alone in indulging in politics

Voting, mutual back-scratching, coups and charismatic leaders - it's everywhere in the animal world
Crisp sales are in decline - but this tasty trivia might tempt back the turncoats

Crisp sales are in decline

As a nation we're filling up on popcorn and pitta chips and forsaking their potato-based predecessors
Ronald McDonald the muse? Why Banksy, Ron English and Keith Coventry are lovin' Maccy D's

Ronald McDonald the muse

A new wave of artists is taking inspiration from the fast food chain
13 best picnic blankets

13 best picnic blankets

Dine al fresco without the grass stains and damp bottoms with something from our pick of picnic rugs
Barcelona 3 Bayern Munich 0 player ratings: Lionel Messi scores twice - but does he score highest in our ratings?

Barcelona vs Bayern Munich player ratings

Lionel Messi scores twice - but does he score highest in our ratings?
Martin Guptill: Explosive New Zealand batsman who sets the range for Kiwis' big guns

Explosive batsman who sets the range for Kiwis' big guns

Martin Guptill has smashed early runs for Derbyshire and tells Richard Edwards to expect more from the 'freakish' Brendon McCullum and his buoyant team during their tour of England
General Election 2015: Ed Miliband's unlikely journey from hapless geek to heart-throb

Miliband's unlikely journey from hapless geek to heart-throb

He was meant to be Labour's biggest handicap - but has become almost an asset
General Election 2015: A guide to the smaller parties, from the the National Health Action Party to the Church of the Militant Elvis Party

On the margins

From Militant Elvis to Women's Equality: a guide to the underdogs standing in the election
Amr Darrag: Ex-Muslim Brotherhood minister in exile still believes Egypt's military regime can be replaced with 'moderate' Islamic rule

'This is the battle of young Egypt for the future of our country'

Ex-Muslim Brotherhood minister Amr Darrag still believes the opposition can rid Egypt of its military regime and replace it with 'moderate' Islamic rule, he tells Robert Fisk
Why patients must rely less on doctors: Improving our own health is the 'blockbuster drug of the century'

Why patients must rely less on doctors

Improving our own health is the 'blockbuster drug of the century'