Venus to "Eclipse" Sun for First Time in 122 Years (Forwarded)

Sky & Telescope
Cambridge, Massachusetts

Contacts:
J. Kelly Beatty, Executive Editor
617-864-7360 x143,

Alan MacRobert, Senior Editor
617-864-7360 x151,

Press Release: May 24, 2004

Venus to "Eclipse" Sun for First Time in 122 Years

On Tuesday, June 8th, the planet Venus will glide directly across the face of
the Sun. No one alive today has seen Venus "transit" the Sun -- it last happened
in 1882 -- and astronomers around the world are eagerly awaiting this rare
event. Only one other transit of Venus will occur this century, eight years from
now on June 6, 2012.

Think of a transit as a miniature eclipse of the Sun. During a total solar
eclipse, the Moon completely blocks the Sun's disk for just a few minutes. But
even though Venus is much larger than the Moon, it's also about 100 times
farther away, so it can cover only a fraction of the solar disk. As Venus makes
its long-awaited crossing, it will appear as a perfectly round black dot slowly
moving across the Sun's face. The most interesting aspects of this 6-hour-long
event will occur when Venus enters and exits the outer edge of the solar disk,
each passage taking about 20 minutes to complete. For example, as it starts to
leave the Sun the planet's silhouette may look pear-shaped, creating what's
called the "black-drop effect."

For observers in eastern and central North America, the Sun rises on June 8th
with the transit already well under way. The entire event will be visible from
Europe, central/eastern Africa, the Middle East, and Asia (except the Far East).
The table below
[http://skyandtelescope.com/aboutsky/...le_1259_1.asp] lists
representative North American cities for which the transit will be in progress
at sunrise, grouped by time zone. Transit times for cities in Europe, Africa,
the Middle East, and Asia are also available at
http://skyandtelescope.com/observing...cle_1021_3.asp

Warning: The Sun is dangerous to look at directly without a safe solar filter.
Staring at it can cause serious eye injury and even blindness.

Fortunately, there are many easy ways to watch the transit safely. If you have
keen vision, Venus should appear just large enough to be barely visible as a
tiny black dot as it crosses the Sun. But when looking at the Sun you must use a
safe solar filter, such as a #13 or #14 welder's glass or special "eclipse
glasses" designed for solar viewing. Filters that are not safe, though sometimes
recommended in error, include smoked glass, stacked sunglasses, metallized candy
wrappers, and compact discs. While these will darken the Sun, they may still
transmit enough invisible infrared radiation to damage your eyes.

Binoculars or telescopes can be used to view the transit, even if they are not
equipped with a safe solar filter. Do not look directly into the eyepiece or the
telescope's finder, but instead place a white card behind the eyepiece and
project the Sun's image onto it. (The small, dark-glass disks that screw into
the eyepieces of some older scopes are not safe and should be discarded. The
Sun's light and heat, concentrated by the telescope, may shatter them suddenly.)

A guide to June's transit of Venus and viewing the Sun safely appears in the
May/June issue of Night Sky magazine, now available on newsstands. To obtain a
PDF of this article via e-mail, media representatives should contact Marcy L.
Dill (617-864-7360 x143; ). More detailed articles,
exploring the historical significance of transits and how to photograph them,
appear in the February, May, and June issues of Sky & Telescope magazine.

For more tips on how to view the Sun safely, see "Safe Solar Observing",
http://skyandtelescope.com/observing...cle_1260_1.asp

Related Articles:

* The Transit of Venus: Where to See It
http://skyandtelescope.com/observing...cle_1021_1.asp
* Safe Solar Observing
http://skyandtelescope.com/observing...cle_1260_1.asp
* Solar Filter Safety
http://skyandtelescope.com/observing...ticle_94_1.asp
* Solar Filter Suppliers
http://skyandtelescope.com/observing...icle_101_1.asp
* Photographing the Transit of Venus
http://skyandtelescope.com/howto/ima...cle_1255_1.asp
* Reanimating the 1882 Transit of Venus
http://skyandtelescope.com/observing...cle_1187_1.asp

Andrew Yee wrote in message m...
Sky & Telescope
Cambridge, Massachusetts

Contacts:
J. Kelly Beatty, Executive Editor
617-864-7360 x143,

Alan MacRobert, Senior Editor
617-864-7360 x151,

Press Release: May 24, 2004

Venus to "Eclipse" Sun for First Time in 122 Years

On Tuesday, June 8th, the planet Venus will glide directly across the face of
the Sun. No one alive today has seen Venus "transit" the Sun -- it last happened
in 1882 -- and astronomers around the world are eagerly awaiting this rare
event. Only one other transit of Venus will occur this century, eight years from
now on June 6, 2012.

Think of a transit as a miniature eclipse of the Sun. During a total solar
eclipse, the Moon completely blocks the Sun's disk for just a few minutes. But
even though Venus is much larger than the Moon, it's also about 100 times
farther away, so it can cover only a fraction of the solar disk. As Venus makes
its long-awaited crossing, it will appear as a perfectly round black dot slowly
moving across the Sun's face. The most interesting aspects of this 6-hour-long
event will occur when Venus enters and exits the outer edge of the solar disk,
each passage taking about 20 minutes to complete. For example, as it starts to
leave the Sun the planet's silhouette may look pear-shaped, creating what's
called the "black-drop effect."

For observers in eastern and central North America, the Sun rises on June 8th
with the transit already well under way. The entire event will be visible from
Europe, central/eastern Africa, the Middle East, and Asia (except the Far East).
The table below
[http://skyandtelescope.com/aboutsky/...le_1259_1.asp] lists
representative North American cities for which the transit will be in progress
at sunrise, grouped by time zone. Transit times for cities in Europe, Africa,
the Middle East, and Asia are also available at
http://skyandtelescope.com/observing...cle_1021_3.asp

Warning: The Sun is dangerous to look at directly without a safe solar filter.
Staring at it can cause serious eye injury and even blindness.

Fortunately, there are many easy ways to watch the transit safely. If you have
keen vision, Venus should appear just large enough to be barely visible as a
tiny black dot as it crosses the Sun. But when looking at the Sun you must use a
safe solar filter, such as a #13 or #14 welder's glass or special "eclipse
glasses" designed for solar viewing. Filters that are not safe, though sometimes
recommended in error, include smoked glass, stacked sunglasses, metallized candy
wrappers, and compact discs. While these will darken the Sun, they may still
transmit enough invisible infrared radiation to damage your eyes.

Binoculars or telescopes can be used to view the transit, even if they are not
equipped with a safe solar filter. Do not look directly into the eyepiece or the
telescope's finder, but instead place a white card behind the eyepiece and
project the Sun's image onto it. (The small, dark-glass disks that screw into
the eyepieces of some older scopes are not safe and should be discarded. The
Sun's light and heat, concentrated by the telescope, may shatter them suddenly.)

A guide to June's transit of Venus and viewing the Sun safely appears in the
May/June issue of Night Sky magazine, now available on newsstands. To obtain a
PDF of this article via e-mail, media representatives should contact Marcy L.
Dill (617-864-7360 x143; ). More detailed articles,
exploring the historical significance of transits and how to photograph them,
appear in the February, May, and June issues of Sky & Telescope magazine.

For more tips on how to view the Sun safely, see "Safe Solar Observing",
http://skyandtelescope.com/observing...cle_1260_1.asp

Related Articles:

* The Transit of Venus: Where to See It
http://skyandtelescope.com/observing...cle_1021_1.asp
* Safe Solar Observing
http://skyandtelescope.com/observing...cle_1260_1.asp
* Solar Filter Safety
http://skyandtelescope.com/observing...ticle_94_1.asp
* Solar Filter Suppliers
http://skyandtelescope.com/observing...icle_101_1.asp
* Photographing the Transit of Venus
http://skyandtelescope.com/howto/ima...cle_1255_1.asp
* Reanimating the 1882 Transit of Venus
http://skyandtelescope.com/observing...cle_1187_1.asp


Last couple of times a transit occured, it was in 1874 and 1882. This
century it's 2004 and 2012. Is there some particular reason why these
transits seem to come in pairs 8 yrs apart? Or is this just some
weird coincidence?

In message , Igor
writes

Last couple of times a transit occured, it was in 1874 and 1882. This
century it's 2004 and 2012. Is there some particular reason why these
transits seem to come in pairs 8 yrs apart? Or is this just some
weird coincidence?

The 8 year gap is because 13 Venus years roughly equal 8 Earth years, so
the planets are in the same positions. I'd guess that the reason pairs
of transits are separated by a variable gap of over a hundred years is a
much more subtle effect of this, but I'd like to know more!
--
Save the Hubble Space Telescope!
Remove spam and invalid from address to reply.

Jonathan Silverlight wrote:

In message , Igor
writes

Last couple of times a transit occured, it was in 1874 and 1882. This
century it's 2004 and 2012. Is there some particular reason why these
transits seem to come in pairs 8 yrs apart? Or is this just some
weird coincidence?

The 8 year gap is because 13 Venus years roughly equal 8 Earth years, so
the planets are in the same positions. I'd guess that the reason pairs
of transits are separated by a variable gap of over a hundred years is a
much more subtle effect of this, but I'd like to know more!

You're right so far.

The 13:8 ratio means that Venus is in inferior conjunction 5 times in
8 years, or to put it another way, Venus's synodic period is very nearly
8/5 years. So each inferior conjunction (which is when transits can
happen) occurs 3/5 of the way around the ecliptic from the previous one.

Now imagine a 5-pointed star circumscribed by a circle (the ecliptic).
The points of the star are spaced 360/5 or 72 degrees apart. If you
number them 1-5 going counterclockwise, then the conjunctions happen
at locations 1, 4, 2, 5, 3, and back to 1 after eight years. OK so far?

If the ratio of the orbital periods were *exactly* 13:8, then the five
points of the star would stay fixed. But it's not. (Aside: you have
to use *mean* orbital elements, not osculating, and you really should
use the "sidereal" period.) Venus's orbital period is 0.615196 Julian
year; Earth's is 1.000017. In eight sidereal Earth years (8.000139
Julian years) Venus makes 13.004219 revolutions. That extra 0.004 rev
means that the inferior conjunction happens somewhat earlier than the
one 8 years before, or a little further west in ecliptic longitude.
In effect the five-pointed star is slowly rotating clockwise.

Now the transits of Venus can happen at only two longitudes, around the
nodes of Venus's orbit (or the line where the two orbital planes
intersect). This line is moving too, but very slowly. So now you can
see what happens: As the five-pointed star rotates, one of the points
will come close enough to the line that you can get a transit. And
usually the tolerance is wide enough that you get *two* transits before
the point has moved on. Then, around 120 years later, when the star
has rotated 36 degrees, another point on the other side of the star
hits the other side of the line, and you get transits in December
instead
of June. And so it goes.

-- Bill Owen
hits the line
is

Jonathan Silverlight wrote in message ...
In message , Igor
writes

Last couple of times a transit occured, it was in 1874 and 1882. This
century it's 2004 and 2012. Is there some particular reason why these
transits seem to come in pairs 8 yrs apart? Or is this just some
weird coincidence?

The 8 year gap is because 13 Venus years roughly equal 8 Earth years, so
the planets are in the same positions. I'd guess that the reason pairs
of transits are separated by a variable gap of over a hundred years is a
much more subtle effect of this, but I'd like to know more!

Actually, I think I figured this one out. Venus, Earth, and the Sun
are roughly in the same relative positions every 8 years, but
sometimes Venus misses the sun by passing above it and sometimes below
it. I realised this when I noted that the two transits occuring 8
years apart would not take the same exact path across the Sun's disk.
This is basically the same thing that occurs roughly every 28 days
between Earth, Sun, and the Moon, but we do not have monthly lunar
eclipses for the very same reason.