the brightness of the sky - Smithsonian Institution

4 LIGHTING OF THE SKY - MOORE AND ABBOT 3 meters, and from the report of. A set of solar altitude or time observations are taken to construct an air-mass-time curve. The slight change in December is probably caused by the blackening of the surfaces. the manganin bands are very little changed.

In March, 1918, an investigation was made at the Hump Mountain Observatory of the sensitivity of the pyranometer to various. A brief description of the rest of the spectrobolometric apparatus used for this test will be given. The spectrum is reflected by a plane speculum metal mirror onto a concave speculum mirror, which brings the spectrum into focus in the plane of the pyranometer strips.

The pyranometer's thermocouple causes the galvanometer needle to deflect, and this reflects a ray from an auxiliary source. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 7 Observations under clear skies.– The greater part of the observations. The main importance attached to these observations in the present discussion has to do with the overall intensity of the celestial radiation and the effect of the sun's altitude or air mass on it.

The column headed 'Pyrh'. gives the solar calories measured on a surface perpendicular to the path of the ray, while 5* is the same reduced to a horizontal surface, or.

NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT d o

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NO. 4 THE BRIGHTNESS OF TPIE SKY MOORE AND ABBOT 11

NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 1

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NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 15

Omitting the two fog observations (October 8 and February 2) and taking a general average of the remaining twenty-four Hump Moun-. From two sets of observations taken under low and apparently foggy conditions at Mount Hump it appears that the middle zone has about a normal percentage, but that the upper zone has increased and the lower zone has correspondingly decreased. The above ratios are obviously very considerably affected by the position of the sun in a given area at the time of observation, as will be shown in the discussions of observations taken in different azimuths.

Although the above relationship applies to the total amount of radiation received from the three 30° zones, it must not be inferred that equal areas of these zones in the different azimuth clouds transmit unequal intensities. To do this, the pyranometer was mounted bodily on an equatorial mount, as is used for pyrheliometers. The threads where the equatorial mount screws into its base were left slightly loose, and the angles were marked so that the whole apparatus could be rotated about a vertical axis through a known angle.

The plane of the pyranometer strips was directed exactly at right angles to the sun's rays, as determined by a spot of light. The metal thimble limiting exposure to a light cone of 60° arc was placed over the glass hemisphere. In all cases the small disc to shade the direct rays of the sun was in place vertically above the strips, even where the instrument pointed to the quarters of the sky away from the sun.

NO. 4 THE BRIGHTNESS OF THE SKY — MOORE AND ABBOT I9

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RADIATION FROM A CLEAR SKY AT 30° AND 19° SUN AT CALAMA

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4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 25Comparison of whole sky M2 and gilfj.— By averaging 64 values Comparison of whole skyat M2 and gilfj.— By averaging af 64 values. This is probably due to a clearing of the sky as the sun gets further from the horizon. The radiation from the area next to the sun is much greater than for other equally sized areas of the sky, e.g.

On January 24, 1919, a series of sky observations were made at Calama, which may turn out to be something interesting and illustrate what just happened. Starting with the sun at about 9° elevation, the measurements alternated between an arrangement of 30° areas facing the sun but with the sun shadowed.

OBSERVATIONS ON CLOUDED SKIES

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OBSERVATIONS AT DUSK

On September 7, the pyranometer, with the auxiliary apparatus, was taken down the mountain side north of the observatory in a. forest of beech and black birch. In a bed of ferns that covered the instrument, but with an open space directly above. Observations were taken on the whole sky and sun two hours before the instrument was moved to the forest, but after that the sky became almost cloudy.

At the end, observations were made of the whole sky, but there was so much variation that the readings were off. With better conditions, the readings showed far less variation among themselves than on previous days. This time the leaves had begun to fall and many that remained on the trees had turned yellow.

Below are the times for the center of each group, the average calories for the group, an estimate of total sky and solar radiation, and the ratio of observed radiation under the trees to estimated radiation above them. Estimated all-sky radiance for September 7 is based on tower observations, following grove observations; for September 12, solar observations with the pyrheliometer and all-sky observations made before the pyranometer was moved to the grove; for October 2nd from pyrheliometer measurements taken at different air masses in the afternoon and total sky observations taken in the morning with the pyrheliometer, using the sum of the two values at the corresponding altitudes of the sun. The galvanometer was placed in a tree about forty yards away, and the ammeter and other therapeutic devices were conveniently placed near by.

On the former sky it was very cloudy and snow fell lightly, so that a thin film of fresh snow covered the patch of old snow. Readings were taken in sets of five, except for a set of four taken at the end with the instrument pointed at the apex. Sets g and h are taken to give an idea of the comparative uniformity of the different parts of the sky.

There was an interval of about forty minutes between / and g to allow recording of a holograph.

NO. 4 THE BRIGHTNESS OF THE SKY MOORE AND ABBOT 33 February 22

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The next day, however, we used sunlight and a cone of skylight 30° around it, so that the compensation could only come from the rest of the sky, this external radiation amounting to perhaps one-thirtieth, or perhaps one the twentieth of that. accepted by the holster. Groups b, c, d and show that the radiation from the snow was fairly uniform over a considerable area. Indeed, the highest readings are found at c, where the instrument was pointed 30° east of the position of b, at which the most direct reflections from the sun were measured; but this is probably because the snow of c was slightly cleaner than that of b.

But we can get an approximation of the total radiation reflected to the point by considering the effect of observing the entire snow hemisphere, or in other words, if we did not use a thimble, it is assumed that this is possible without the entrance of scattered light. The rays that are most directly reflected will be the most effective as they come closest to normal incidence on the pyranometer strips. We can now estimate the total rebound by multiplying the thimble results by 4 (the derivation of this ratio is given in the thimble section).

SUMMARY