Moon Shadow Research Articles

Ionospheric Effects during the Solar Eclipse of March 7, 1970

IT has been predicted that the supersonic movement of the Moon's shadow during a solar eclipse will generate internal gravity waves in the atmosphere1. The bow wave so produced by the Moon's shadow will be manifested by travelling ionospheric disturbances (TIDs). During the March 7 eclipse last year some of the observations at Fort Monmouth, NJ, were devoted to TID observations using measurements of the integrated electron content of the atmosphere, obtained from the polarization of the beacon signal from the ATS-3 satellite at 137.35 MHz. Detection of the predicted motions, by variations in the electron content, depends on achieving sufficient precision in relation to an unknown effect. We therefore carried out an error analysis from more than 1,100 single measurements taken in the afternoon of March 7 (after the eclipse had finished. We found a maximum error of ±5.5 polarization degrees inherent in our new automatic system for the data points in Fig. 1, each of which is averaged over twenty-eight single digitized data points taken within four seconds at intervals of three minutes. This averaging is necessary and compensates for the spin effect on polarization. The same maximum error was observed experimentally when the fluctuations of electron content were studied during ionospherically quiet days. The data of Fig. 1 were obtained using an effective geomagnetic component of constant value, neglecting the unknown variations of this factor. The data were analysed for undulations by taking the differences between actual data and their running mean taken over nine data points, over twenty-seven minutes, in other words. The maximum error for these differences is then ±11 polarization degrees. For reasons of improved reliability, we assumed a maximum error of ±15 degrees or ±3.5×1011 electrons cm−2 for the variations in content deduced during the eclipse (Fig. 2).

Visual Requirements for Landing on the Moon

An analysis was conducted to determine some of the visual requirements for landing a shuttle spacecraft on the surface of the moon and subsequently achieving rendezvous with the command module in orbit. The major questions investigated were: (1) Is the astronaut capable of perceiving moon landmarks while in orbit in order to detect and identify a desired landing area? (2) Can the astronaut perceive the command module from the moon during all phases of its orbit from lunar horizon to entering the moon's shadow? To evaluate these questions, the properties of the moon landmarks were examined in terms of size (visual angle subtended), contrast, and velocity of the moving imagery. In addition, stellar magnitude and luminance data were calculated for the command module and the earth as seen from the moon for different phase angles and albedos. Finally, the effects of glare on the discrimination of the command module were discussed.

THE EFFECT OF THE SOLAR ECLIPSE OF JUNE 30 UPON THE MORNING TWILIGHT AT PALOMAR OBSERVATORY

The solar eclipse of June 30, 1954, was not visible at Palomar Mountain ; by sunrise the penumbra of the moon's shadow had moved well across the country far to the northeast of southern California. At the time of the beginning of morning twilight at Palomar, however, the sun, rising at a point in the north central United States and contributing to Palomares twilight, was still approximately three-quarters eclipsed. It was expected that the sun, so eclipsed, would produce a darker than usual dawn at Palomar. To determine this effect quantitatively, the brightness of the sky, both at the zenith and at the celestial pole, was measured at brief intervals during the early period of morning twilight with a nightsky meter, mounted just east of the dome housing the 48-inch schmidt telescope with which the authors were photographing fields for the National Geographic Society-Palomar Observatory Sky Survey.

Survey by Moon's Shadow

Survey by Moon's Shadow

Eclipse expedition to study ionosphere

What happens to the nebulous ionosphere, the all‐important radio‐reflecting layer high above the Earth, when the Moon's shadow “punches a hole” in it during an eclipse of the Sun? This will be investigated on May 20, 1947, by the Army Air Forces‐National Geographic Society Eclipse Expedition at Bocayuva, Brazil. Because the ionosphere is created by ultraviolet radiation from the Sun, which makes air‐molecules in the upper atmosphere electrically conducting, scientists are interested in knowing what happens to the ionosphere when this radiation is cut off suddenly during an eclipse.Study of the ionosphere is important because of the increasing use of long‐distance communication by radio. High‐frequency radio signals travel around the Earth in a series of giant bounces between the ground and the reflecting layers of the ionosphere, which range from about 50 to 250 miles in altitude. When these layers disappear or are disrupted, long‐distance communication by radio fails or is garbled.

THE TOTAL SOLAR ECLIPSE OF JULY 9, 1945

At sunrise on the morning of May 28, 585 b.c., the tip of the moon's shadow touched the earth on the equator at a point in the Pacific Ocean in longitude 105° W, moved northeast over what are now Honduras and Cuba, crossed the Atlantic, entered Europe near southern France, and skirted the northern boundary of the Mediterranean. Toward evening it swept across Asia Minor in the vicinity of Ankara, where it chanced that the Medes and Lydians were engaged in battle. The eclipse was not wholly unexpected, however, for according to Herodotus, the Milesian predicted to the Ionians that this change would happen, fixing beforehand this very year, in which the change did occur. The Lydians and the Medes, when they saw that it was night instead of day, ceased from fighting, and on both sides endeavored more anxiously to obtain peace. The renown which Thaies acquired from the prediction of this eclipse would seem to be more than he deserved, if he could do no better than fix the year of its occurrence. Not only was the eclipse total but what was more important for Thaies* reputation, it happened at a decisive moment in history, two circumstances which Thaies with all his wisdom could scarcely have foreseen.

Astronomical Topics

The Total Solar Eclipse of Aug. 31.—Further bulletins from Science Service, Washington, indicate that several observing parties obtained a fair measure of success. The U.S. Naval Observatory party at Limerick, Maine, was able to carry out its full programme, though thin cirrus cloud was passing. The General Electric Company sent up an aeroplane at Concord, N.H., which reached perfectly clear sky; the moon's shadow was seen for three minutes before and after totality. The Harvard party at Portland, Maine, had a completely clear sky. The Greenwich party at Parent obtained a satisfactory photograph of the flash spectrum extending from Hα to the H and K calcium lines; but the attempt to obtain the infrared portion failed.

Astronomical Topics

Application of Sound Films to Timing Solar Eclipses.—The Annual Report of Mount Wilson Observatory for the year 1930–31 refers to an ingenious method of obtaining the exact time and duration of totality in the very brief total eclipse of April 28, 1930. The Mount Wilson party was stationed at Honey Lake, California, which is at lat. 40° 8′ 20″N., long. 120° 15′ 30″W., altitude 4000 feet. Moying pictures with sound records were taken by the Fox Movietone News. The observers gave audible tune signals about the time of totality, which were recorded in conjunction with the film snowing the progress of the eclipse. The time of mid-totality was thus recorded as 19h 5m 51.4s U.T., which was 1.7s earlier than the prediction; the duration of totality was recorded as 1.2s, which was 0.2sless than the prediction. It is worthwhile noting that totality could frequently be timed in this manner, even if the weather is cloudy. At Colwyn Bay in 1927 the sky was densely overcast, but the passage of the moon's shadow on the clouds at the beginning and end of totality was plainly visible. As the seconds of Greenwich time were audible, distributed by wireless from Daventry, such a record would have enabled the times to be deduced at least to the nearest second.

Moon's Shadow at Eclipse Photographed

Moon's Shadow at Eclipse Photographed

The Moon's Shadow in Relation to the Earth on June 29

The Moon's Shadow in Relation to the Earth on June 29

OBSERVATIONS OF THE TOTAL SOLAR ECLIPSE, SEPTEMBER 21, 1922

Twenty minutes before totality there was a distinct change in the general illumination of the landscape, the light becoming of a somewhat livid color. The sky had turned a darker blue, the reduction in brightness being rather more noticeable in the western than in the eastern sky. Venus was brilliant. Twelve minutes before totality the sky had assumed a yellowish color round the horizon. This gradually faded with increasing elevation, and from an altitude of about 20 degrees to the zenith it was dark blue. As totality advanced the difference in illumination between the western and eastern sky, noted above, became much more pronounced. Four minutes before totality the light was very striking in its color, objects in the landscape appearing much as if viewed at normal times through yellowish (Fieuzal) sun glasses. Thereafter the light became much more livid, and by two minutes before totality white objects assumed bluish and purplish tints. About ten second before totality I took up a position facing southwards, holding a camera to be used in an attempt to photograph the Moon's shadow in the sky. The sky was darkening at a rapidly increasing rate, but it was not until within about four seconds of totality that I could discern any gradation of intensity from east to west across the southern sky. About 3.0 or 2.5 seconds before second contact the gradation was certainly noticeable, and I believe that a snapshot taken then might have recorded it. Thereafter, however, the light rapidly diminished and the gradation disappeared so that 1.5 or 1.0 second before second contact was called the sky illumination appeared to have reached a minimum and totality to have commenced.

Next Year's Total Solar Eclipse (September 21, 1922)

IN September of the coming year there will be a very favourable total eclipse of the sun, and several expeditions are preparing to proceed to stations on the narrow track of the moon's shadow. This eclipse is a member of an important family, from the point of view of solar physics, because one of its predecessors was the memorable eclipse of August 18, 1868. On this occasion the astronomical equipment was enriched for the first time by the use of the spectroscope for such work, and the important discovery of the gaseous nature of the solar prominences during that eclipse was the forerunner of very rapid advances in solar research. The track of the shadow was slightly to the north of the coming one, and passed over North-East Africa, India, Java, and North Australia. It was in India that the memorable observations were made.

The Deflection of Light during a Solar Eclipse

IN discussing the effects of atmospheric refraction during solar eclipses Prof. Anderson disregards the shallowness of the effective layer of air as compared with the diameter of the moon's shadow. Unless the sun be very near the horizon, a line of sight drawn from the centre of the umbra to a point in the corona will remain within the umbra right through this layer. This consideration vitiates the method of solution adopted by Prof. Anderson, and consequently its results. On reading his first letter (NATURE, December 4, 1919) I was struck by the ingenuity of his explanation, more especially as I believe he under valued the amount of the angular deviation arrived at on his theory through taking the sun's radius to be half, instead of a quarter of, a degree. In view of the importance of the subject, a fuller investigation seemed to be required. I hope soon to publish a note giving the complete solution of the problem, and may therefore confine myself here to a statement of the result, which is quite fatal to Prof. Anderson's explanation. I take the altitude of the sun to be 45° and the maximum fall of temperature 4°; the figures given may easily be modified to suit other conditions. I further assume the most favourable distribution of temperature, which is that adopted by Prof. Anderson, when the line of maximum fall of temperature is parallel to the edges of the moon's shadow and independent of altitude. Two stars at a distance of three solar, diameters from each other might then show an increase in apparent distance owing to refraction amounting to the 240,000th part of a second of arc. If the diminution of the temperature effect with altitude be taken into account, this figure should be divided by 4.

The Deflection of Light during a Solar Eclipse

THERE can scarcely be a downward rush of cold air in places deprived of the sun's radiation during an eclipse as suggested by Prof. Anderson. This would happen only if the upper layers of the atmosphere were cooled more than the lower, and if the cooling were sufficient to bring the temperature-gradient near to the adiabatic. As it is, however, the effect of an eclipse should be to cool the lower layers more than the upper, and so to decrease the temperature-gradient. Moreover, if cooling caused convection movements, we should have upward currents as well as downward, and a development of cumulus clouds would result from the passage of the moon's shadow.

The Influence of Small Changes of Temperature on Atmospheric Refraction

The Paper is an investigation of the possible deviation of the light from a star near the sun due to the temperature changes in the atmosphere produced by the passage of the moon's shadow across the earth during an eclipse. It is shown that while the actual displacements from this cause vary widely for slight differences in the assumed conditions, they are always negligibly small compared with the effects observed at the last solar eclipse.

Wireless Telegraphy and Solar Eclipses

JUST before the beginning of the war in 1914 the Radiotelegraphic Committee of the British Association, which was appointed at the Dundee meeting in consequence of a suggestion by me, had, completed arrangements for certain observations to be made on the strength of radiotelegraphic signals on the line of totality of the 1914 solar eclipse which passed through Russia. These arrangements were rendered useless by the outbreak of the war. On May 29 of this year a total solar eclipse will be visible in North Brazil, and it seems very desirable that any eclipse expeditions sent out to observe it should be provided with wireless telegraph apparatus, and should arrange to receive, and also to send, signals to other stations before, during, and after the passage of the moon's shadow.

The Forthcoming Total Solar Eclipse, August 21

OWING to the great strides made in the study of the physics of the sun, the importance of the occurrence of a total eclipse of the sun is not so great as it was towards the latter end of last century. Nevertheless, there are still some problems to be solved, the data for which can only be obtained on these occasions, thus necessitating the organisation and dispatching of observers to several stations lying on the path traced out by the cone of the moon's shadow as it sweeps over the earth's surface.

The Solar Eclipse of April 17: The Annular Eclipse as Observed near Chavenay, France

THE recent eclipse of the sun was of interest from several points of view, but chiefly in exact path of the moon's shadow and the duration of totality. It was well known that there existed a great deal of uncertainty as to both these items, the calculations depending on the different values employed.

American Observations of the Total Solar Eclipses of 1900 and 1901.1

IN the year 1900 the moon's shadow swept across the southern portion of the United States of America, travelling from New Orleans, through Georgia, South and North Carolina, and leaving this continent at Cape Henry, in Chesapeake Bay. After traversing the Atlantic Ocean the shadow crossed Spain, and cut the African coast at Algiers.

Our Astronomical Column

LOCAL CONDITIONS FOR OBSERVATION OF THE TOTAL SOLAR ECLIPSE, 1901, MAY 17–18.—A pamphlet has been received containing information for observing parties and summaries of the climatological conditions along the track of the moon's shadow during the total solar eclipse in May 1901. The work is the report of a committee of the society, Koninklijke Natuurkundige Vereeniging in Nederlandsch-Indië, appointed at the request of the Government at Batavia.