630315Eclipse3_p4

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Eclipse Bulletin No 3 cont'd.

4. TIMING THE FLASH SPECTRUM

    For a brief instant at the beginning and end of totality, the narrow dark
Fraunhofer absorption lines of the sun's spectrum are reversed, that is, they shine
out as brighb lines, the more intense backgxvund of the light from the surface of
the sun having been cut off. If binoculars are used to obsene the flash spectrum,
a grating (4000 lines or more to the inch) should be placed over one objective and
welder's glass over tkt other. The observer can then watch the approach of totality
through one lens and be ready to observe the flash spctrum through the other. A
telescope can be used with either a grating over the objective or a spectroscope
attachment at. the eyepiece while the finder, with welder's glass over the dbjebtive,
can be used to watch the approach of totality The flash spectrum should be timed
as accurately as possible and the observer's impressions as to the, intensity of the
lines, etc. should be recorded. A clock drive on the telescope would be an advantage.

5. TIMING THE SHADOW BANDS

The shadow bands are fleeting shadows that race along the ground before ans
after the main shadow. The direction of these bands and their line of motion differ
before and after totality and with the position of the observer relative to the,line
of totality. They can be best seen against a white background. Therefore, two white
sheets should be spread on level ground- one to observe the bands before totality
and the other for after totality. A team of three is required - one timekeeper and
two observers.  The procession of shadow bands may last from ten to fifteen, seconds.
The observers call "Time!" the instant the bands appear and again when they disapp-
ear, the timekeeper recording the times to the nearest second.  One observer has a
supply of sticks or bamboo rods which, as the bands appear, he quickly places on
the sheet to indicate their dir ection and line of motion.  The other observer con-
centrates on the width of the bands, whether they are straight or wavy, their colOur
and any other characteristics, The rods `are left in position so that, with the aid
of a compass, their true direction can be determined unhurriedly after totality is
over.  All such details as to time, duratIon, direction, will be of value from every
point on the eclipse path.  It will be seen that for this project the team will need
to practice to get operations down to a routine.

6. METEOROLOGICAL DATA

    There are two reasons for recording weather' conditions - (1) because of the.
effect of'the weather on observations made, and (2) because of the effect of the'
eclipse on atmospheric conditions. For the former, one needs no special eqiupment,
simply recording the observing conditions (seeing, transparancy, cloud cover) during
the period of the eclipse. These should be recorded at five-minute intervals and;
in addition, whenever a change in conditions occurs. For the latter, Equipment con-
sisting of wet and dry thermometers, barometer, anometer, photometer, etc., should
be in operation for a fUll hour before first contact and a full hour again after
fourth contact.  During these tqo one-hour periods, readings should be made at five-
minute intervals. During the eclipse - the partial phases and totality readings
should be made at two-minute intervals. An electric clock, with sweep second hand,
would be sufficiently accurate for recording the readings. If possible, the equip-
ment should be installed on the previous day and the team should make readings for
the same period as on eclipse day. Not only will this furnish comparison graphs
but will give the team the practice needed to carry out operations smoothly on
eclipse day.

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