Why did the path that BBC showed for the Venus Transit not match the
live images?
Did they get it wrong?
If you don't understand the question, have a look at the
BBC news item on the
Venus Transit. There is an animation half way down the page. This shows Venus traversing
from about clock position 8:00, to about clock position 4:30, in a straight line.
Those of us that watched it saw Venus traverse from about clock position 7:00, to about clock
position 4:30, in a curve. Why is this?
Firstly for those who did not see it, or did not see enough on the news, here is an
animated version of the path taken. Times are in GMT which is one
hour behind BST. This is done from Guide but it ties up
with the live coverage shown throughout by Sky News Interactive.
The BBC didn't get it wrong, but they didn't say that they had drawn the sun so that it was
always in the same orientation. This does not happen when we view the sun. When we view the heavens,
unless we are looking through an equatorial mounted telescope, we use the Alt/Azimuth coordinate
system. We have a vertical going through our viewing position and by rotating round we can view East,
West, South, North and anything in between. These 360 degrees of the compass rose are the Azimuth of
the system. The Alt of the system is the elevation above the horizon. The Sun rises in the east, and
climbs higher in the sky as it moves round through the south. As it does so we turn to face it and just
look a bit higher up in the sky. The TV camera following the transit does the same. However the Sun is
not traveling around us in a circle, with an axis through our vertical, (the Sun doesn't move at all, it
is the Earth rotating, but it is simpler to explain as a Sun motion). The Sun is travelling on the ecliptic
which is roughly in the orbital plane of the Earth. Because the Earth's axis is tilted with repect to the
orbital plane, the path of the Sun across the sky is an arc of a circle and the axis is the axis of the
Earth's rotation.
Because we are not viewing with respect to this coordinate system but using our local
Alt/Azimuth system the effect is for the Sun to be tilted. Astronomers used to equatorial mounts will be
familiar with this rotation or twisting of the field of view. The amount of tilt or twist varies according
to where the sun is. This can be seen on the animated gif as the Sun's latitude and
longitude are drawn in, you can see the Sun rotating in orientation (not to be confused with rotating on its
axis which it also does). If this rotating was undone then the path would be
the straight line of the BBC flash animation. Note that at the end of the transit the orientations are
about the same. This is because the coordinate systems line up on the meridian and no apparent rotation is
in effect. Since the transit ended shortly after noon BST, or before noon GMT, it was close to the meridian at
that time and any apparent rotation would be minimal.
|
