Safe viewing of the Transit of Venus Biman Basu 1

Global visibility of ToV 5/ Unlike the transit in 2004, this year’s transit will begin long before sunrise in India. So only part of the transit will be visible.

Why the Sun is dangerous to look at The Sun is an extremely hot and bright object. It emits 40% visible light; 58% infrared (heat) radiation; and 2% ultraviolet (UV) radiation. The visible light that provides us with colour vision represents just a small part of the electromagnetic spectrum. The UV and infrared radiations are invisible to the human eye, but can have dangerous effects on the retina. 3

The Sun in many colours Sun in visible light Sun in infrared Sun in ultraviolet 4

The human eye 5

Structure of the retina 6

The risk of directly viewing the Sun 7

Sun induced damage to the eye After just one direct viewing of the Sun, a person can attain a retinal ‘burn’ creating a blind spot known as ‘solar retinopathy‘, which often results in impairment or loss of vision. Solar retinopathy is damage to the eye's retina, particularly the macula, which is the region of keenest vision, from prolonged exposure to solar radiation. It usually occurs due to staring at the Sun, or watching a solar eclipse or a planetary transit without eye protection. 8

Solar retinopathy 9

How the Sun damages the retina Exposing the retina to direct sunlight triggers a series of complex chemical reactions within the light-sensitive rod and cone cells. The products of these reactions impair the cells' ability to respond to light, and in extreme cases can destroy them. Depending on the severity of the damage, an affected observer experiences either a temporary or permanent loss of vision. 10

Retinal damage is painless Both photochemical and thermal retinal injuries occur without the victim's knowledge, because there are no pain receptors in the retina, and the visual effects do not occur for at least several hours after the damage is done. Although it is frequently claimed that the retina is ‘burned’ by looking at the Sun, retinal damage appears to occur primarily due to photochemical injury rather than thermal injury. 11

Damage to the cornea Staring directly at the Sun can also damages the cornea, leading to a condition known as ‘photokeratitis’, which is a painful inflammation of the cornea. Typically, the lens of the adult human eye absorbs some UV rays to protect the eye. But, prolonged exposure to the Sun overwhelms these natural defences, often leading to photokeratitis. This condition usually heals on its own in about a week. 12

Photokeratitis 13

Safe viewing of the transit of Venus Filters made from certified aluminised Mylar sheets, black polymer sheets, or dark welder’s glass (no. 14), which cut off at least 99.99% of the Sun's visible light and 100% of the harmful UV, are safe for viewing the progress of the transit. Use of pinhole camera or small mirror, although good for viewing solar eclipses, will produce fuzzy image of the Sun in which Venus may not be clearly visible. Projection of the Sun's image through a telescope with a convex eyepiece produces the sharpest image and is totally safe for watching the progress of the transit. 14

Safe solar filters 15 CAUTION! Never put on the eclipse goggles and then look through binoculars or a telescope !

Watching through a telescope When watching the transit through a telescope, the Mylar filter must be fixed in front of the objective. Never use "eyepiece" solar filters! 16

Solar filters for telescope 17

The projection method The best way for amateur astronomers to observe the position of Venus during transit is to project the image of the Sun on a white card using a small telescope. CAUTION! NEVER LOOK AT THE SUN DIRECTLY! 18

DON’Ts while observing transit of Venus Don’t attempt to observe the Sun with naked eyes. Never look at the Sun through a telescope or binocular without a proper filter, placed in front of the objective; not behind the eyepiece. Don’t use smoked glass, colour film, sunglasses, exposed black-and-white film, or polarising filters, as they are not safe. Don’t look at a reflection of the Sun from water, even coloured water. 19

20 Watch the transit safely! It’s a once-in-a-lifetime opportunity to see one of Nature’s rarest spectacles. HAPPY VIEWING!

Thank You! 21