MERCURY’S TRANSIT
ACROSS THE SUN
Dr.
N.Rathnasree*
The month of
May 2003 is full of excitement for the sky enthusiasts. The most
exciting event will be the transit of Mercury across the disc
of the Sun that will be visible on the morning of the 7th of May.
Compared to the transit of Venus, an event no human being currently
alive would have seen, Mercury’s transit is much more frequent.
The last time this was visible from some regions on Earth had
been during November 1999. That was visible only from parts of
Australia. It was not visible from India. The last transit of
Mercury, visible from India, had been during November 1993. The
transit or passage of a planet across the disc of the Sun is a
relatively rare occurrence. As seen from Earth, only transits
of Mercury and Venus are possible. On the average, there are 13
transits of Mercury in each century. In comparison, the transits
of Venus occur in pairs with more than a century separating each
pair. More specifically, pairs of them occur separated by 8 years,
but the pairs are themselves separated by either 105.5 years or
121.5 years.
Phenomenon
What exactly
is a transit anyway? It is one of the many games of hide and seek
the celestial bodies play out for our sake. It is not very different
from an annular eclipse of the Sun we will see this same month.
An annular eclipse of the Sun takes place when the Moon is at
a relatively farther distance from the Earth in its elliptical
orbit around the home planet. At such times the disc of the Moon
does not cover the disc of the Sun exactly and at the time of
the maximum eclipse, an outer ring of the Sun remains uneclipsed.
The discs of
the planets Mercury or Venus, as seen from Earth, are much smaller
than that of the Moon. Therefore, they make no more than a small
black dot when they move in front of the face of the Sun. Usually
it takes more than a couple of hours for this small dot – like
Mercury or Venus - to cross the face of the Sun. With every transit,
depending on the geometry involved, this dot may walk a different
path across the face of the Sun. But one thing will remain unchanged
- its overall direction of motion. In the solar system, almost
every motion is anticlockwise. If one works out what this means
in terms of Earth’s spin, Sun’s movement in the sky and the motion
of Moon and the planets Mercury and Venus, one will see that,
invariably, ingress - the point of entry in front of the solar
disc - for Mercury, Venus or Moon, is always from the east and
the exit on the western edge.
The plane of
the Earth’s orbit round the Sun is known as the plane of the ecliptic.
Since we are on the Earth, the ecliptic is the apparent path followed
by the Sun through the stars. The orbits of other planets round
the Sun are tilted at small angles to the ecliptic and hence planets
will usually be either above (north) or below (south) the ecliptic.
The transits of the Sun will occur if the inferior conjunction
occurs within a day or two of the date at which the planet crosses
the ecliptic.
All transits
of Mercury take place within several days of either 8th May or
10th November. Since Mercury’s orbit is inclined seven degrees
to Earth’s, it intersects the ecliptic at two points or nodes
which cross the Sun each year on those dates. If Mercury passes
through inferior conjunction at that time, a transit will occur.
During November transits, Mercury is near perihelion and exhibits
a disc only 10 arc-seconds in diameter. By comparison, the planet
is near aphelion during May transits and appears 12 arc-seconds
across. However, the probability of a May transit is smaller by
a factor of almost two. Mercury’s slower orbital motion at aphelion
makes it less likely to cross the node during the critical period.
November transits recur at intervals of 7, 13, or 33 years while
May transits recur only over the latter two intervals. The next
Mercury transit will be a November transit again and it will not
be till the year 2006.
So much for technicalities.
But what exactly is in store for us this May? First let us concentrate
on the upcoming Mercury transit. If one wishes to have a look
at Mercury before the transit, it can be spotted on the 2nd of
May. Towards early morning hours on that day, it has a close encounter
with the crescent Moon straying barely 2 degrees away from it
in the sky. Mercury is rather elusive, fluttering close to the
Sun, always. The best times to spot it are when it is either close
to Venus or the crescent Moon in the evening or morning skies.
If you do glimpse
it in the morning skies on the second of May, make an appointment
to meet it again on the 7th. You can see it in broad daylight
on this day. On the 7th of May, 2003, exactly at 10:42:56 IST
it will pop across the eastern limb of the Sun at first contact,
as a small black dot, traverse the entire face of the Sun, westwards
from this point, and disappear on the western edge, at 4:01:46
PM IST.
These are contact
times as seen from the centre of Earth. The actual contact times
will differ, by a very little, from place to place. There are
four important points of time during the transit that astronomers
would like to measure. These are called 1st, 2nd, 3rd, and 4th
contact. The 1st contact, which is very hard to observe, is the
point in time when Mercury first touches the disc of the Sun.
The second contact is the time at which Mercury is fully engulfed
within the Sun’s disc. The third contact is when the disc of Mercury
just begins to leave the face of the Sun and the 4th contact is
when the disc of Mercury has just completely left the Sun’s disc.
Every location on Earth had its own contact timings. The illustration
-I clarifies these concepts. The actual path of Mercury across
the disc of the Sun will not really appear so straight. The illustration
– II shows the way the motion would appear as Mercury moves across
the disc of the Sun. India is at a very favourable position for
viewing both the transits. In fact the entire astronomical spectacle
can be seen from any part of India. The duration of the May 7,
2003 transit is - 5hours and 18 minutes. Here are the timings
for the transit -Contact I -10:42:56 IST, Contact II - 10:47:24
IST, Greatest - 13:22:23 IST, Contact III - 15:57:29 IST and Contact
IV - 16:01:46 IST
Illustration
-III is a view of Mercury just before the third contact, during
the Mercury Transit of November 1999. If you have a telescope
of your own and an accurate clock such as a WWV radio or GPS receiver,
you can do these timings and send them in to the Association of
Lunar and Planetary Observers (ALPO). They would appreciate getting
your observations. The observations will be used to refine orbital
figures and the diameter of the Sun.
Sir Edmund Halley,
the noted astronomer in the 18th century, was the first to realize
that transits could be used to measure the Sun’s distance. Kepler’s
laws gave relative distances between all the planets and the Sun.
But the absolute distances were not known. The 1761 and 1769 expeditions
to observe the transits of Venus gave astronomers their first
good value for the Sun’s distance. Captian Cook had observed a
transit of Venus from Tahiti in June,1769 and from Mercury Bay
in New Zealand in November of the same year during his exploration
of the coastline of New Zealand. In India too, there had been
some observations of transits. But, unfortunately, there were
no rigorous measurements of scientific value. The earliest recorded
use of telescope in India was by an Englishman, Jeremiah Shakerley.
He was one of the earliest followers of Kepler and viewed the
transit of Mercury in the year 1651 from Surat. He could, however,
time neither the ingress nor the egress.
Preview
A transit will
start and end at slightly different times when viewed from different
places on the Earth. By timing the events from various places
on the Earth and the "parallax" involved in these measurements,
the distance to the Sun can be determined. More accurate methods
are available now, but careful measurements in the 18th and 19th
centuries gave distances to within 1% of that currently accepted.
The entire transit will be visible from all of Asia, except the
extreme eastern portion, Africa except the western portion, Europe
except Portugal and western Spain, Greenland except the southern
portion, and most of the Indian Ocean – that is, about half the
globe will be able to see this event together. A caution – DO
NOT TRY TO LOOK AT THE DISC OF THE SUN DIRECTLY. THIS COULD LEAD
TO BLINDNESS. The only safe way of viewing this event will be
to project the image of the Sun on to a screen and view the projection.
One will need a moderate aperture telescope to be turned towards
the Sun and its output projected on to a screen, preferably some
darkening provided around the projection area.
Using such a
projection equipment at other times you can view sunspots on the
disc of the Sun. Try with the sunspots first to get a feel for
setting up the projection apparatus and then make your appointment
with the Sun and Mercury on the 7th! Just as an inconsequential
aside, recall that no transits can occur for the outer planets.
However, if you happen to hover close to any of the outer planets,
you may sometime view transits of the Earth and any other planet
nearer the Sun than them.
On 11 May 1984,
for instance, the Earth was in transit of the Sun had it been
viewed from Mars. The Earth took about 8 hours to go across the
face of the Sun with the Moon about 6 hours behind, so that for
a while both bodies would have been seen as black dots crossing
the Sun. If you had been observing from Mars!
Coming back to
Earth, let us remember that this is a preparation for a more exciting
event a year away. You will not wish to miss the transit of Venus
coming up on June 8, 2004. Because the orbit of Venus is larger,
and Venus is much closer to us when it is between us and the Sun,
the tilt of its orbit almost always causes the planet to cross
above or below the Sun’s disc, making Venus transits very rare
events. This is only the sixth transit of Venus since the telescope
was invented.
The 2004 transit
of Venus will be the first in 121 years. Remember that Venus transits
occur in pairs. The last time this had happened in 1874 and 1882.
And now, the event next year will be followed by another Venus
transit that will be observable on June 6, 2012. So, let us enjoy
these events together, from May this year to June 2004 and the
one 8 years ahead! It is not just transits, there seems to be
plenty of watercress around this event - an annular solar eclipse
on the 31st visible from many parts of India - the map (illustration
–VI) shows the regions from where it will be visible. Only the
very edge of the eclipse is visible from India and that too only
from some regions in northwest India. There is another exciting
event to watch out for this same month - an occultation of Venus
by the thinnest of crescent Moon - visible in the morning hours
of 29th of May. As it happens, the occultation will take place
after sunrise for most locations in India. Even so, it should
be possible to view this if good, clear skies exist towards the
eastern horizon. This will be one time we can appreciate the fact
that Venus is actually visible in broad daylight, if only we know
where to look for it!
Although this
event will be in daylight hours, it will be worth watching for,
if only to experience the thrill of seeing Venus in broad daylight!
A " Public Sky Watch" for viewing the Transit of Mercury,
on the 7th of May, will be arranged by Nehru Planetarium, New
Delhi. The Planetarium will also have an exhibition describing
many aspects of the May 7 transit and the upcoming historical
Venus transit next year. A workshop will be conducted on the 3rd
of May at 10:30 AM at the Planetarium to explain to anyone interested
in the importance of these transits.(PIB Features)
*Director,
Nehru Planetarium, New Delhi.
**Please
download the attached illustrations from www.pib.nic.in
Illustration – I
Illustration – II
Illustration – III
Illustration – IV
