Some Introductory Concepts
hello venustransit
I’d
like to know what on earth is a transit
help!!!
- Tarang
I am very happy you asked this question, as it lets
us begin at the
beginning
:-) The word Transit is used in two ways in
Astronomy - we will be
concentrating
on that shade of meaning which refers to the passage of
one
celestial body in front of another. In the current case that we
are interested
in –
Planetary transits are a bit like such annular eclipses, except that the coverage of the SUn's surface is even smaller in this case - such a small dot of the Sun's disk is covered by the Planets Mercury or Venus, when they move in front of the SUn, that there is no noticeable decrease in the amount of sunlight reaching us - one would not even get to know that there is a Planetary transit, unless we know that it is going to take place, and then set out to observe it. The upcoming Venus Transit is on the 8th of June, 2004. But, it is time for us to get started, now. - Rathnasree, Nehru Planetarium, New Delhi
1) Can we see Venus Transit with naked eyes or we have to use some special device? 2)Can we observe it from our house? waiting for ur reply from, - Harsha, student of kerala school
DEAR
HARSHA,
NEVER
TRY TO OBSERVE THE TRANSIT EVEN THE SUN FOR ANY MOMENT IN NAKED EYE,
AS
DOING SUCH THING YOU MAY LOOSE YOUR SIGHT FOREVER.
SOME
SUGGEST THAT THE RAYS WHICH ARE COMMING FROM SUN DURING SUN-RISE OR
SUN-SET
ARE NOT SO HARMFULL TO OUR EYES, BUT MY DEAR FRIEND MAKE IT VERY
MUCH
CLEAR THAT VIEWING SUN IN NAKED EYE FOR ANY MOMENT IN ANY CONDITION IS
VERY
MUCH HARMFULL TO YOUR EYES EVEN WHEN THE SUN IS REALLY BELOW THE
HORIZON
AS IN THE CASE OF SUN-RISE OR SUN-SET.
Eye safety is a prime consideration when you're viewing the solar surface (with or without Venus in transit). Always use a safe solar filter or an indirect projection technique, even when the Sun is low on the horizon. Remember, always use a safe solar filter to view the event, even when the Sun is low in the sky.
IT
IS BETTER TO OBSERVE THE EVENT BY PROJECTING THE SUN ON A WHITE SURFACE
THROUGH
A TELESCOPE OR OTHER DEVICE.
BUT
IF YOU WISH TO SEE THE SOLAR DISC DIRECTLY THROUGH THE TELESCOPE THE
PROPER
SOLAR FILTER SHOULD BE IN USE.
SO
TO OBSERVE THE "VENUS TRANSIT" WE ARE TO OBSERVE THE SOLAR DISC,CERTAINLY
NOT IN NAKED EYE, EITHER IDIRECTLY OR WITH PROPER SLOLAR FILTER.
AND
IF YOU PROPERLY EQUIPPED AND PROTECTED YOU CAN OBSERVE THE EVENT FROM
YOUR
HOUSE,BUT IT IS BETTER TO ENJOY THE EVENT IN GROUP WITH SOME EXPERT'S
HELP.
- DIPAK
Just out of curiosity I would like to know that in
past or future,
was/is
there any event when transit of mercury and venus occured at same
time?
Is this possible? Rohit Belapurkar
I do not think this is possible as seen from
Earth
- the node crossing times for Mercury are around May and
November
while for Venus are around June and December - close enough -
may
be precession of perihelion for the orbits will make them
coincide
sometime - give or take a few million or more years -
anyway,
all this is without checking up - maybe some one will have
some
more answers. Rathnasree
(Following the discussion on definition of
transits) -
When
one object passes in front of another there are two terms that may be used
for
the event. If the nearer object (the object in front) is the larger of the
two
and completely covers the more distant object such that it disappears
completely
then the event is an occultation, i.e., the farther object is occulted.
Examples include occultations of stars or planets by the Moon. If
the
nearer object is smaller than the more distant one then we see the disk of
the
smaller object moving across the "face" of the more distant object (if
the contrast
allows it to be visible). This event is a transit.
So technically a total solar eclipse is an occultation of the Sun by the Moon. An annular solar eclipse -- as you point out -- is actually a transit of the Moon across the Sun. The difference with Venus or Mercury -- or even a satellite or weather balloon -- is merely in the relative sizes of the objects.
Of course, I would never suggest total solar
eclipses be referred to as total
solar
occultations! Everyone understands what we mean when we say "solar
eclipse".
But when we are comparing different events then sometimes it is
confusing
when there is inconsistency in the usage of the terms. When you
display
drawings that show how the different events occur and compare them then
I
think it is especially important.
I
hope this helps a little and doesn't cause more confusion!
Mike
Simmons
Iam harshita gupta and would like to know about ingress and egress
- Harshita
Harshita, Ingress refers to the entry of Venus on
to the disk of the SUn and
egress to its leaving the Sun's disk.
More precisely - 1st contact =External Ingress 2nd contact=Internal
Ingress 3rd contact = Internal Egress and 4th contact = External Egress.
1st contact, which is very hard to observe, is
the point in time when
the Planet first touches, apparently, the disk of the sun. 2nd
contact is the time at which Venus is fully engulfed within the sun's
disk and is internally tangent to the disk. 3rd contact is when the
disk of Venus just begins to leave the face of the sun, and 4th
contact is when the disk of Venus has just completely left the sun's
disk and is externally tangent to it. While inside the disk of the
Sun, Venus moves across at a rate of about 4 arc minutes /hour. The
greatest transit happens when the centers of Venus and Sun would
appear to have the minimum distance as seen from the center of the
Earth.
http://rathnasree.htmlplanet.com/transit.htm
has a section on the simple understanding of AU determination from
transits which has these terms defined with a figure. Many websites
on transits will, of course, define these quantities. Rathnasree