Safe viewing of the Transit
of Venus
Biman Basu
bimanbasu@gmail.com
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Global visibility of ToV 5/6.6.2012
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.
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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.
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The Sun in many colours
Sun in
ultraviolet
Sun in visible
light
Sun in infrared
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The human eye
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Structure of the retina
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The risk of directly viewing the
Sun
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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.
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Solar retinopathy
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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.
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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.
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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.
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Photokeratitis
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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.
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Safe solar filters
CAUTION! Never put on the
eclipse goggles and then look
through binoculars or a telescope!
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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!
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Solar filters for telescope
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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!
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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.
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Watch the transit safely!
It’s a once-in-a-lifetime
opportunity to see one of
Nature’s rarest spectacles.
HAPPY VIEWING!
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Thank You!
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