On June 8, 2004 the DOT
the first Venus transit in 122 years at high angular resolution in the
G band and in the Ca II H line. The DOT observers also used full-disk
cameras in white light and Halpha.
The DOT missed the ingress because the Sun rose over La Palma
only after the start of the transit.
The DOT has a parallactic mount so that the transit path is
straight. Celestial North is up.
The G band (so named by Joseph von Fraunhofer in 1817,
consisting of molecular CH lines at wavelength 430.5 nm in the blue
part of the solar spectrum) samples the solar photosphere from which
most sunlight escapes. This apparent "surface" (made of hot gas, not
one to walk on) shows granules and intergranular lanes which are
produced by convection in the outer layers of the Sun.
The Ca II H line at wavelength 396.8 nm (violet, once-ionised
calcium atoms) samples the low solar chromosphere, at about 500 km
above the white-light (or G-band) surface. The pattern shown in the
Ca II H line contains very bright patches made up by magnetic
concentrations (much stronger than the Earth's magnetic field) and
weaker patches produced by sound waves which permeate the solar
atmosphere as well as the solar interior (the Sun rings like a giant
church bell but at much lower frequencies: 0.001 - 0.01 Hz).
The Halpha line at wavelength 656.3 nm
(red, highly excited hydrogen atoms) samples the high solar
chromosphere at about 1000 km above the white-light surface. Active
regions with very strong magnetism appear bright in this spectral line
(except for sunspots which remain somewhat dark).
The Venus atmosphere becomes bright just outside the solar limb
in the logarithmically-scaled G band egress movie
image. This remarkable
brightening, known since Mikhail
observation of the 1761 transit, is due to refraction in the Venus
atmosphere which bends the transmitted sunlight toward us. Similar
brightening is seen in a companion Ca II H
where one also observes the
chromospheric forest of solar spicules extending outward from the
The near-limb "black drop" phenomenon (which hampered the
measurement of the ingress and egress times in many historical
determinations of the solar parallax) is not seen at egress in our
Ca II H movie
because the DOT is a
high-resolution telescope while the Sun shows only slight limb
darkening in this scattering-dominated spectral line. It is
momentarily seen, but very weak, in the third frame of the
logarithmically-scaled G band egress movie. It becomes much more
pronounced at lower resolution.