Transcript Document

History of the Transit
A transit of Venus is a rare event, occurring in pairs eight years apart
separated by more than a century due to the tilt in Venus’s orbit compared
to that of the Earth (Figure 1). Kepler accurately predicted the transit of
Venus in 1631. However, it was not visible in Europe that year, and he
incorrectly predicted that Venus would barely miss the sun in 1639. The
young English scientist Jeremiah Horrocks discovered Kepler’s mistake.
Horrocks and his friend William Crabtree were the only two people to
observe the 1639 transit. They got a sense of the immensity of the solar
system. Crabtree was so astounded by the event that he even neglected to
take actual scientific data.
The Transit of Venus
Kayla Gaydosh, Bryn Mawr College ‘05
Keck Northeast Astronomy Consortium Fellow
Advisor: Prof. Jay M. Pasachoff
8 June 2004
Figure 6: Me at Mt. Hollomon in
Greece during the Transit.
Figure 5: Image courtesy of Jay M. Pasachoff, David Butts ‘06,
Owen Westbrook ‘06, and Joseph Gangestad ‘06.
Greece Expedition
Figure 2: Drawings by Thobern Bergman of
the “black drop” effect.
For centuries a transit of Venus was the best method available for the
determination of the distance to the sun, also known as the Astronomical
Unit. Transits of Venus can be used to determine the distance to Venus
from its parallax, requiring observations to be taken at different parts of
the world at the same time. This idea led to dozens of international
expeditions for the 1761 transit. But Thobern Bergman in 1761 reported
that the silhouette of Venus was joined to the dark background exterior to
the sun (Figure 2). This dark "black drop" meant that observers were
unable to determine the time of contact to better than 30 seconds or even 1
minute. The black-drop effect thus led to uncertainty in Venus’s contacts
and thus the Astronomical Unit.
The most famous of all the Venus transit expeditions was that of Captain
Cook, who was sent by British Admiralty to Tahiti for the observations.
His later wanderings around New Zealand and the eastern coast of
Australia are thus spin-offs of astronomical research. The observations by
Cook and his astronomer, Charles Green, clearly show the black-drop
effect. Cook also correctly mentioned the existence of an atmosphere
around Venus (discovered at the 1761 transit) but incorrectly attributed
the inaccuracy of the “black drop” effect to that atmosphere.
Figure 3: Photo of 1882 transit from
the U.S. Naval Observatory Library.
The 1872 transit was observed in the Indian
Ocean and Australia. During the 1882
transit, which was visible in the western
hemisphere, the U.S. Naval Observatory
took the image in Figure 3, which is one of
the 11 surviving photographic plates from
the observations they coordinated of that
transit. The “black drop” effect was still a
major problem during the 19th century
observations.
The Williams College Transit of Venus Expedition Team traveled to the Aristotelian University of Thessaloniki, Greece.
Ground-based observations were carried out at the University using their 20-cm refractor with our Apogee and SBIG CCD
cameras. Photos were taken with a Nikon F5 camera (Figure 5). A team of students including myself was sent to Mt.
Hollomon outside of Thessaloniki to observe the transit for weather insurance, but unfortunately it clouded out (Figure 6).
Steven Souza was able to observe the third and fourth contacts with the Carroll spar 5” refractor at Williams College.
(b) Study its morphologic and photometric evolution over time prior to
second contact and after third contact.
(c) Confirm with space-based imagery the assessment that ground-based
historical reports of “black drops” are due to the convolution of the
instrumental and atmospheric PSF in conjunction with the limb darkening.
(d) Investigate the detectability of “aureola” during photospheric transits.
OBSERVATION
TRACE imaged the transit at high temporal cadence during (and flanking) the
planet's crossing of the solar limb and while on the heavily limb-darkened
portion of the solar disk. All images were obtained in TRACE's "White
Light" (WL) configuration, providing spectral sensitivity in the wavelength
range from 1200-9600 angstroms. During the transit, Venus's angular
diameter was approximately 58.2 arcseconds (12,104 km at 0.289 AU).
IMAGES and MOVIES
Figure 7: Ground-based
image from Greece at
approxmately second
contact.
Figure 8: Isophotes on a
TRACE image with a limbdarkening function removed.
Figure 9: Ground-based
image from Greece of
Venus during transit.
Ground-based Observations
The ground based images taken both in Greece and Williamstown have been partially reduced (Figures 7 & 9). These will
give insight onto the properties of modern telescopes by inspection of the “black drop” effect. Analysis of final reduced
images will give the characteristics of the point-spread function of telescopes. A qualitative analysis of these images shows
that there is a decreased “black drop,” but it is not entirely eliminated. The results of the 2004 expedition will provide better
preparation for the 2012 Transit of Venus.
Figure 10: Ingress (above) and egress (below) images in sequence taken by the TRACE spacecraft in white light showing the scattering of light in the Cytherian
atmosphere and no black drop effect. The atmosphere can still be detected below (see arrow) in the second image from the right. (G. Schneider, Univ. of Arizona)
Acknowledgments:
- Glenn Schneider, Steward Observatory, University of Arizona, Tucson, AZ
- Committee for Research and Exploration of the National Geographic Society
as grant provider for the Williams College Expedition to Greece.
The study of the Transit of Venus in 2004 is an alternative test for observing
methods, strategies and techniques that can assist in the detection and
characterization of extrasolar terrestrial-type planets as they transit their stars.
Primary objectives of observations of our team, headed by Jay Pasachoff and
Glenn Schneider, with NASA’s TRACE spacecraft:
(a) Image the circum-Cytherian “aureola” (sunlight scattered by aerosols
and refracted in the backlit Cytherian atmosphere) with high spatial
resolution and image stability.
Figure 4: Expedition team at the
birthplace of Aristotle.
Figure 1: Illustration of the orbits of
Venus and Earth showing the
possibility of a transit of Venus.
Transition Region and Coronal Explorer
(TRACE) Spacecraft Results
To view/download the movies go to Glenn Schneider’s Website:
http://nicmosis.as.arizona.edu:8000/ECLIPSE_WEB/TRANSIT_04/TRACE/TOV_TRACE.html or
link to them from our site at http://www.transitofvenus.info.
Working with Glenn Schneider (University of Arizona’s Steward
Observatory), we produced the first photometric results from the ingress and
egress imaging sequence taken in TRACE's WL band pass (Figure 10). I
assisted in the making of movies for both sequences with all frames aligned
along the solar limb. Venus-centered movies were also made since the
evolution of the morphological and photometric characteristics of the light
scattered and refracted by the Cytherian atmosphere is better studied by
“fixing” the position of Venus in each frame.
APERTURE PHOTOMETRY
Aperture photometry was executed using the
IDP3 (Schneider and Stobie 2002) image
analysis software. For each image of Venus we
measured three concentric, radially nonoverlapping rings, of which all three are centered
on Venus (Figure 11). The central ring had a
distance of 58.63 pixels from the center of
Venus, setting it 45 km above the “surface” of
Venus (the optically thick cloud layer in the
Cytherian atmosphere). Radially adjacent regions
interior to and exterior to the central points were
measured for proper background estimation and
to estimate the uncertainties in the measures.
Figure 11: Aperture
photometry circles on a grid.
Relevant data obtained for each aperture are the X/Y locations of the aperture
center, the total number of pixels used to compute the mean value (pixels
only partially contained in the aperture are weighted linearly by pixel area),
the total flux, the maximum value of the flux in any pixel, the median flux in
a pixel, and the one-sigma dispersion in flux. From this the background
subtracted ring flux will be computed along with its uncertainty. The ring
surface brightness will be characterized as functions of azimuth angle, the
distance from the center of Venus, the distance from the point on the limb
along a radius joining Venus and the heliocenter, and distance from the
closest point on the solar limb. These data are currently ready for analysis and
are the main objective in the weeks ahead.