Information sheet n°07 :
How to observe on June 8 in order to get a
useful measure
J.-E. Arlot (IMCCE)
I. Introduction
When having understood how to observe the Sun, with what instruments and
how to make safe observations of Venus in front of the Sun, one should be
prepared to make useful observations and measurements in order to calculate the
astronomical unit either with our simplified formulae and the measure made by a correspondent, or by sending
your measurement to the center of calculation which
will send you back the result from your measurement and the result from all the
measurements received.
In order to be able to send your measurement, you should register first
by connecting to the address below :
Attention, be careful to register as many times as you have several
instruments : one measurement will need one registration. One registration will
allow you to send only one measurement (one measure of the four contacts).
II. Observation of the contacts
The observation itself consists in the determination of the precise time
of the contact between Venus and the limb of the Sun. Attention, this timing
should be made in Universal Time obtained thanks to GPS or thanks to the clock
by phone or by any radio signal. It is not necessary to modify your watch, just
note precisely the difference between your watch and the Universal Time. At
last, note that the measure must be made with an accuracy better than one
second of time. Four contacts have to be measured :
-
The first contact is an exterior contact
(labelled 1 on the figure 1) between Venus and the disc of the Sun (near
impossible to observe because, when you will see Venus, it will be too
late !). Only the observations made with a Hα filter and recorded on images
could allow to measure the first
exterior contact.
-
The second contact is an interior contact (labelled
2 on the figure 1) between Venus and the limb of the Sun (easy to
observe : as soon as Venus enters the disk of the Sun, be prepared to
measure the instant of the second contact (it takes about 20 minutes for Venus
to go from the first contact to the second contact); attention to the
« black drop », an effect due to the diffraction which leads Venus to
be sticked to the limb of the Sun with some kind of
chewing-gum! (see fig. 2).
-
The third contact (labelled 3 on the figure 1)
is an interior contact between Venus and the Solar limb (the easiest to observe
since we observed Venus moving in front of the Sun: attention to the “black
drop” here also)
-
The fourth contact (labelled 4 on the figure)
is an exterior contact between Venus and the Solar limb (easy to observe when
Venus definitively leaves the Solar disk)
Fig. 1 Description of the
contacts
III. Methods to measure the times of contact
a) Visual method
You observe Venus and try to determine the time of contact. When you
think this is time either you switch on a button to record this time or you
give a signal to an assistant who can accurately read the time (one second of
time is necessary) on a time keeper or a clock. The measure of this time will
be delayed of about 1 or two tenth of a second because of your “personal
equation” (your reaction time to record an event), but his delay can be
determine independently to the observation.
b) The recording of images
If a video camera, a CCD camera or a webcam is
available, you can record a series of images which have to be also all referred
to the Universal time or other timescale with a known shift to the Universal
time. The analysis of such images will allow you to determine the times of
contact by interpolation.
Fig 2 The Black Drop
IV. The observation of least distance from Venus
to the Solar limb
If you record images, well referred with respect to the Universal time, all along the transit of
Venus, they could be used to compute the Earth-Sun distance by another method
than by observing the contacts. Therefore, despite possible bad meteorological
conditions at the beginning or at the end of the transit of Venus, you will be
able to participate to the computation of the Earth-Sun distance.
For this purpose, your images must allow us to measure the distance
between Venus and the Solar limb (see fig. 3). The edge of the Sun, and Venus
must be simultaneously recorded on your images, even if Venus is far from the
edge (see fig. 4 and 5) . If you carry out this kind of observations, you could
send us your images for analysis (send first an e-mail to images@imcce.fr in order to get
instructions). In this case, we also ask you to send us your agreement to keep your
images – which will be transformed in a standard format – in a data base in
order to be used in a next future for educational purposes. You will have access to the whole
data base.
Fig. 3 Image of Venus in transit for a given time:
the least distance to the edge is the observational data useful for the
analysis.
Fig. 4 Unusable images : we cannot measure the
least distance to the Solar limb
Fig. 5 Useful images :
the least distance to the Solar limb can be measured.
Note
concerning the refraction:
The refraction is an atmospheric phenomenon which reveals to us the stars
in the sky higher than they are, actually. Thus, at the time of sunset, the
observers still see the Sun whereas it is already below the horizon! This
phenomenon of raising of the stars is as stronger as the star is lower on the
horizon (i.e. stroner as the atmosphere crossed by
the light is thick). Thus, a star observed at the zenith is not affected by the
refraction.
We thus see that the Sun that we observe and on whom we measure theVenus distance to its center,
is, actually, lower in the sky than it does appear to be. No importance, will you
think, since only relative measurement is necessary. In fact, since the
refraction varies according to the elevation of the Sun in the sky, and since
the Sun is relatively large, the effect of the refraction is stronger in bottom
of the Sun than in top! It is noted besides that at the time of the sunset, the
Sun appears to be flattened: it is the differential refraction. It will thus be
understood that our measurement of distance on the Sun is likely to be affected
by this effect if the observation takes place when the Sun is low on the
horizon. However,
the method described above allows a relative measure which is not affected
by the differential refraction if some precautions are taken:
- to use the values of the theoretical diameters of Venus and the Sun non
affected by the differential refraction (values given above)
- to measure the Venus diameter which is used to us as unit on the line
which joins the center of the Sun to the center of Venus (Venus may have an ellipsoidal form)
- to avoid the use of an observation made when the Sun is too low on the
horizon (less than 20° in height), an additional correction being then necessary,
the differential effect between the top and the bottom of the Sun being too
large.