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Observe Eclipses!
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Solar Eclipse Observing
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.
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 well. 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 advantageous here. 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. Viewing 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 filters.
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