April 14th, 2004|
8th June 2004 the planet Venus will cross the Sun's disk as
seen from Earth. It will cut into the solar disk at about
5:20am in the morning and will pass off the other limb at
about 11:23am. The last occasion on which this happened was
1882, over a century ago.
A transit of Venus is like a solar eclipse,
but instead of the Moon being in line between the Earth and
Sun it is the planet Venus. You can hardly fail to notice
a solar eclipse because the Moon, being about the same apparent
size as the Sun, blocks out its light.
Venus, on the other hand, looks very much
smaller from Earth and so you would have to be specifically
observing the Sun to see the small disc of Venus passing across
its disk. You would also have to wait a very long time. Solar
eclipses occur every year or so somewhere on Earth, but you
could have been observing the Sun for the last century and
still not have seen a transit of Venus.
Venus appears very small (Venus is only about
58'' - the Sun is about 1890'') as seen from Earth, so as it
passes across the Sun, it will be just big enough to be seen
without any magnification with the unaided eye through a solar
filter. Using image projection onto a card will also show
the planet's slow progress but perhaps a little to set up
in say, a school playground.
Maddox Brown's painting, in the Great Hall of Manchester's
Town Hall, depicting William Crabtree as an intense,
emaciated puritan, observing the 1639 transit of
Venus with an elaborate equatorial telescopic apparatus.|
To start to understand what is going
on, imagine a circular race track and two runners, Eartha
and Vera. Eartha can do a lap of the track in one minute.
Vera is quite a bit faster, and also has the advantage of
the inside track, she can do a lap in just under 37 seconds.
The question is, if they start off together how long is it
before Vera catches up with Eartha so they are together again?
work out the answer it is convenient if the runners' speeds
are in a whole number ratio, so let us assume Vera can lap
the track in 8/13 minutes (36.92 seconds). That means Vera
is going 13/8 times faster than Eartha, so Vera is catching
Eartha up at 5/8 laps per minute (13/8 minus 1). Vera will
therefore catch up Eartha after 8/5 minutes, ie 1.6 minutes
or 1 minute 36 seconds. Check it out and you will see that
in 1.6 minutes Eartha has gone round the track once plus 3/5
of the track while Vera has gone round the track twice and
3/5 of the track, so they meet up.
Translate runners into planets and minutes
into years and the picture is close to that for the actual
planets, Earth and Venus. It is not quite as simple as that
because the tracks are a bit elliptical rather than circular
and the runners do not go at quite a constant speed. Also
Venus orbits the Sun just a bit faster than 8 thirteenths
of a year, more like 7.997. As we shall see this is a small
but crucial difference.
time of 1.6 years between Sun, Venus and Earth lining up is
called the synodic period (synod = meeting). Fig 1
shows the lined-up positions from 1996 to 2002. You can see
that there is a five spoke pattern. (Ignore for the moment
the dotted line). The planets travel anti-clockwise and each
lined-up position is 3/5 of the circle on from the previous
If we continue the pattern we can see that
the next lining-up after 2002 will be in June 2004 in the
same place as June 1996, 8 years previously. Almost, but not
quite. Since Venus travels a little bit faster than one orbit
in 8/13 of a year it catches up with the Earth a little bit
before the meeting place 8 years previous. Consequently, if
you draw the five spoke pattern 8 years on from Fig 1
it will look like Fig 1 but with the spokes rotated
We can start to see that 8 years might be
a significant interval but it is still not clear how it relates
to transits. For a start we have said that Earth, Venus and
the Sun are lined up every 1.6 years, so why is there not
a transit of Venus every 1.6 years? The answer is there would
be if the orbital planes were exactly aligned. However, the
orbit of Venus is at a small angle (3.4 degrees) to that of
Earth. The result is that, although the planets appear lined
up when looking down on the orbital planes, most of the time
Venus viewed from the Earth will be either above or below
the Sun. For a transit to occur the planets not only have
to line up as in Fig 1 but they have to do it just
in the place where the orbital planes cross, which makes it
a much rarer event.
The same thing happens with a solar eclipse.
Every month the Earth, Moon and Sun are lined up at New Moon.
However the Moon/Earth orbit plane is at an angle to the Sun/Earth
orbit plane and it is only when New Moon occurs at the orbit
plane crossing that there is a solar eclipse. Otherwise, seen
from Earth, the New Moon is either above or below the Sun.
back to Fig 1 the dotted line shows where the orbit
planes of Earth and Venus cross. The crossing line corresponds
to about 7 June and 8 December. We can see that the lining-up
in 1996 was very close to the orbit-crossing line but was
not quite there. Venus was near to the Sun but passed just
south of it. On 8 June 2004 the lining-up will be even closer
to the orbit-crossing line, in fact so close that Venus will
transit the south side of the Sun for some 6 hours from about
5h 15m to 11h 15m UT. Eight years later in 2012 the lining-up
will occur just the other side of the orbit-crossing line
and there will be another transit, this time with Venus transiting
the north of the Sun. Fig 2 illustrates the position
of the Sun seen from Earth relative to the orbit path of Venus
on the three occasions. (The orbit angle has been exaggerated
so Fig 2 is an illustration rather than a precise picture).
After that there is a long wait until the
January 1998 "spoke" in Fig 1 slowly makes
its way round clockwise in eight year ticks until it gets
close to the December orbit-crossing line in 2117. Eight years
later in 2125 there will be another transit when the "spoke"
ticks through to the other side of the orbit-crossing line.
This time the orbit path rises from south to north so the
first transit will be north of the ecliptic and the second
transit south. There is then a long wait while the "spokes"
tick round until the lower one arrives close to the June orbit-crossing
line again. And so the process continues.
On a final note we saw that because the orbit
time of Venus (measured in units of Earth-orbit time, ie one
year) was close to 8/13 it led to 8 year cycles. A closer
approximation to the orbit time of Venus is 243/395 years,
which by similar logic leads to a longer run 243 year cycle.
Peter M Langford, September 1998
will we expect to see?
The two images below show
the area of Earth that can view the 2004 transit of Venus.
The left image shows the start, on the right is the position
at the end.
|This diagram below shows the path Venus will
take across the Sun. It also shows the comparative size of Venus.
Indeed, Venus' disk is about one hundred times smaller than
the Sun, but because it lies closer to us on this occasion, its
disk is bigger than normal.|
|Most astronomical events happen at great distances
from the Earth, so timings tend to apply to anywhere on the
Earth's surface. So accurate timings are essential for your
particular location, all those given on this site are for
Orpington and Bromley.
to view the transit in safety
Firstly, you have to get up early!
The Sun will be low down in the east.
of the Sun onto card is by far the safest method.
Even a modest telescope will show Venus.|
To observe the transit you will need either
a pair of binoculars or a small telescope, two pieces of white
card and some kind of photographic tripod (a solid chair will
With one of the pieces of card cut one hole
in the centre if you have a telescope or two holes if you
have binoculars for the eye pieces. Push the card onto the
telescope or binoculars and fix with masking tape (sellotape
is okay but it is transparent). On binoculars, use the lens
cap (or something else opaque) to block one of the
objectives so that no sunlight can get in, and use the
Mount the binoculars or small telescope onto
the tripod and fix them in such a way that they can be pointed
into the sky towards the Sun. Get someone to hold the other
piece of white card a short distance away from the eye piece
(as shown in the photograph). Slacken the mounting so that
you can move the telescope with ease.
DO NOT UNDER ANY
CIRCUMSTANCES TRY TO ALIGN THE BINOCULARS OR TELESCOPE WITH
THE SUN, BY LOOKING THROUGH THEM.
Move the telescope around until the 'smallest
shadow' has been achieved onto the white card, you will quickly
notice that every time you get the 'smallest shadow' the image
of the Sun jumps across the card. The trick is to lock the
mounting once the image of the Sun can be seen on the white
card. There you have it, the right equipment for the right
AT NO TIME DURING
THE TRANSIT SHOULD THE APPARATUS BE LEFT UNATTENDED - BLINDNESS
IS NO FUN AND THE DAMAGE CANNOT BE UNDONE!!
During the six hour transit you will have
to move the set up in order to follow the Sun. However, because
you are using a small telescope or binoculars you will find
several minutes of comfortable viewing before the apparatus
has to be reset.
For those who do not own a tripod, a high
backed wooden chair is also ideal, but following the Sun may
prove a little tricky.
Of course, if you have a nice sharp image
you can 'peek-a-boo' the camera over the edge of the masking
card to capture images of the transit as it progresses.
All times given
in UT (hour:minute:seconds).
The times of
the transit given below are those for Orpington and Bromley.
at the Sun at any time is potentially dangerous and can result in
serious eye damage or blindness. The safest way to view the Sun is
indirectly using a projection method.
You may view
the Sun directly only through a special filter made for safe solar
viewing. If you are not certain that a filter is approved and safe
or you have any other doubts - DO NOT USE
As you can see from
the list below, transits of Venus across the Sun's disk are very
rare (there are only about 12 per millennium!). We are quite lucky
that two transits are going to happen in the next few years although
the second one is only partially visible from some parts of Europe.
There is a curious
243 year repeating pattern with two transits in December (around
the 8th) 8 years apart. Then wait 121 and half years, then two June
transits (around 7th), again 8 years apart. Then wait 105 and half
years and the pattern repeats again.
Below are four
diagrams showing each of the "contacts" for the 2004 transit
as seen from Orpington.
is a difficult bit to spot because there are no known fixed
reference points on the Sun's surface! Perhaps a few trial runs
before the event may give the less experienced photographic
observer more confidence of finding the right part of the sun's
limb to aim at!|
this point, you might begin to witness the famous "black
drop" effect, where the disk of Venus appears to pull out
a small dark extension from the solar limb. It may be that between
first and second contact several venusian atmospheric phenomena
can be witnessed - dark ones AND light ones.|
wait but it will have been worth it. Watch carefully for the
"black drop" again a short time before internal egress.
- Venus finally pulls away from the Sun. Once off the solar
disk it will vanish into the void - BUT the diligent may witness
some unusual atmospheric events, so be watchful for quite a
while after final contact!|
Black Drop Effect
Many people have
witnessed the Black Drop effect. This happens when the second and
third contacts occur.
Because Venus has an atmosphere,
sunlight gets refracted around the limb of the planet just at those
crucial times. Although under normal circumstances this would be
a bright light effect, during transits the point where Venus makes
contact with the edge of the Sun, appears to grant Venus a little
extra diameter. In fact the opposite is true, most of Venus' size
is slightly compromised in transit, by sunlight leaking over its
limb - what we witness is every sunrise and sunset on this distant
very comprehensive paper on the black drop effect here
at the University of Bologna. (The O.A.S. is not responsible for it.)
Some keen eyed
observers have seen what appears to be atmospheric events during
ingress and egress, so be prepared for anything.
During the transit
there is a fair chance of Venus passing close by a sun spot group.
During this time there is the opportunity to make a direct comparison
between what appears to be "black" sunspots and the silhouette
Below is a simulation.
However, if Venus does indeed pass close to a spot you will certainly
have seen an extremely rare and unusual event!
at Inferior Conjunction
A few hours before
and after the transit, Venus will be in a suitable position for
those with the right equipment to take pictures of Venus' atmospheric
"ring" and any possible evidence of the famous "Ashen
Venus is extremely
close to the sun at its inferior conjunction. It is only at this
time, the "ring" of Venus' atmosphere can be photographed.
The only way this can be done, is by shielding the close proximity
of the sun by a "sunscreen" extending far away from the
telescope. This allows only the light of Venus to enter the telescope,
while completely blocking the sun from doing so.
Venus' Atmospheric Ring
Seen at Inferior Conjunction as Photographed from Table Mountain
was taken on June 20, 1964 using a Kodak III-F Spectroscopic glass
plate and an RG-1 Schott glass filter with the 16-inch cassegrain
telescope operating at f/60. This 0.08 second exposure was made
with a specially designed 'planetary camera', made by the late Rex
Bohannon for Table Mountain. In this frame, the sun is to the immediate
left, only a few degrees. The entire 'ring' of Venus' atmosphere
can be seen.