Solar Eclipse Observing
-Second and third Contacts
Introduction. Totality or annularity
arrives suddenly, and for totality, with striking spectacle.
Second contact marks the beginning of totality or annularity.
For annularity, this occurs when the moon’s trailing limb
reaches the sun’s disk. For totality, it is when the moon’s
leading edge completely covers the sun’s disk. Totality is
further identified as the moment when Baily’s last bead winks
out, or when the brightness of the sun’s photosphere fades to
the level of the adjacent chromosphere and inner corona.
Photograph 10-1 Second contact of the
10 May 1994 annular eclipse from El Paso, Texas. SVHS
image taken through a 68 mm lens with a 5X telextender.
Video taken by John Westfall.
The flash spectrum is seen at second
contact, when the photosphere’s dark absorption lines are
suddenly replaced by the chromosphere’s bright emission lines.
The chromosphere itself, a beautiful pink/red to scarlet color,
is visible near second and third contacts.
During totality, jets of glowing gas,
ranging from red to scarlet, may be seen arching more than
30,000 km (19,000 mi) from the sun’s disk. These are the
prominences, most commonly found in the higher solar latitudes
and in greatest number a few years after a sunspot minimum.
Finally, third contact and the reappearance
of Baily’s beads ends totality or annularity. For annularity,
this occurs when the moon’s leading limb leaves the sun’s
disk. For totality, it is when the moon’s trailing limb
exposes the sun’s photosphere once more.
Contact timings. Timings of the
second and third contacts are considered important and should be
carried out for all central eclipses; different procedures are
recommended depending upon which contact is being timed and
whether the eclipse is total or annular.
Eyepiece projection onto a white screen
offers the best method of timing second contact for total
eclipses, since the photosphere projects prominently while the
chromosphere does not. Visual observations by unaided eye or
binoculars, using safe visual filters, yield adequate results
for general purposes. However, the brightening of the
chromosphere and corona as totality draws near, and the lack of
a clear–cut separation between the photosphere and the
chromosphere, make visual measurements somewhat less reliable
than those made by eyepiece projection.
For annular eclipses both methods seem to
work equally well, although the higher resolution (i.e., the
ability to discern fine detail) afforded by binoculars or
telescopes makes it easier to determine the instant when the
last tiny segment of the moon’s dark limb moves onto the
sun’s face and the annulus becomes complete.
During annular eclipses a few observers
have reported a “black drop” irradiation effect (where the
planet’s limb seems to delay its separation from or hasten its
reunion with the sun’s limb) similar to those observed at
solar transits of the planets Mercury and Venus, where that last
lunar limb segment appears to linger on the edge of the sun,
elongate, then separate suddenly. Watch for it, as it can
complicate contact timings.
Third contact is best timed by the
reappearance of Baily’s beads for a total eclipse. The first
bead’s appearance stands out in stark contrast to the dimmer
phenomena of totality, and presents little difficulty for
eyepiece projection or direct viewing methods.
For an annular eclipse, an optical system
providing the best available resolution is preferred. Observers
should watch for any “black drop” effect at third contact as
Flash spectrum. The flash spectrum
was anticipated, first observed, and named such by Princeton
professor C. A. Young at the total eclipse of 22 December 1870,
who described it in this way:
…the moment the sun is hidden, through
the whole length of the spectrum, in the red, the green, the
violet, the bright lines flash out by hundreds and thousands,
almost startlingly; as suddenly as stars from a bursting
rockethead, and as evanescent, for the whole thing is over in
two or three seconds.
Pogson was first to observe the flash
spectrum during an annular eclipse on 6 June 1872.
Inexpensive diffraction gratings and
spectroscopes are widely available and amateurs with an interest
in more technical observations are encouraged to include the
flash spectrum in their programs.
Chromosphere. This innermost region
of the sun’s atmosphere is primarily of historical
significance, since it was in the spectrum of this light during
an eclipse in 1868 that the element helium was first identified
more than 25 years before it was finally recognized on earth.
The chromosphere exhibits the same red to scarlet color as the
prominences, and should be identified and noted as a part of any
total eclipse observing program, but it is essentially
featureless and warrants only passing attention.
Prominences. What the chromosphere
lacks in structure, the prominences make up for. Stannyan first
described prominences in a letter to Flamsteed following the
eclipse of 1706, but the first detailed descriptions of them
were by the Swedish astronomer Vassinius at the eclipse of 1733
(although he incorrectly believed them to be lunar in origin).
Spanish admiral Ulloa, observing the eclipse of 24 June 1778
from sea, suggested they were caused by sunlight shining through
breaks in the moon’s limb, but they were not widely accepted
as a solar phenomenon until the eclipse of 1842.
Prominences may be relatively quiescent,
persisting for many weeks without significant change, or they
may be violently active, erupting outward as far as three
million kilometers (two million miles) from the sun’s surface.
The more active prominences will exhibit changes from minute to
minute; noting any movement or change is a worthy undertaking.
Observers viewing with telescopes equipped
with eyepiece reticles (system of lines or dots in the focus of
an eyepiece) might wish to measure the position angles of the
prominences arrayed about the sun’s limb. Techniques for doing
so may be found in Chapter 9.
The extent of particularly broad
prominences, especially those which loop about and return
arch-like to the solar surface, may be estimated in a similar
manner. Some reticles have a vertical extension scale as well as
an azimuth circle; these may be used to measure the relative
heights of prominences above the sun’s limb.
Photography. Still and video
sequences around the time of second and third contacts can
pinpoint the times of these events when taken in conjunction
with tape–recorded time signals; video cameras are especially
Flash spectrum spectroscopy utilizing
diffraction gratings and simple spectroscopes can be especially
rewarding. Try the wide selection of fast color films and
low–light video cameras on the market and practice by
attempting to capture spectra of the sun and the full moon to
determine the best system for you. Howerver note that flash
spectrum photography is very challenging!
Specific exposure recommendations for the
chromosphere and prominences may be found in Chapter 17, but it
is wise to bracket exposures somewhat to capture the varied
structure exhibited by the prominences.
dangers. Great care should be taken when timing second and
third contacts to prevent eye damage. Safe visual filters are
required whenever the sun’s photosphere is visible. Flash
spectrum observers using diffraction gratings should be careful
to look only at an angle through the grating and not directly at
the sun. The chromosphere and prominences do not present any
danger from infrared radiation, and may be viewed without
Photograph 10-2 Prominences visible at
the 11 July 1991 total solar eclipse from Cabo San Lucas,
Mexico. Note the distinctive "seahorse"
prominence at the bottom (south). This exposure was
taken through an 80 mm f/8 refractor at prime focus with a
Nikon F3 camera body on Ektar 125 film. Photograph
taken by Doug Berger.