Astronomical phenomena | Previous Top Next

ECLIPSES

Partial eclipses of the sun or moon provide interesting spectacles but afford no opportunity for the seaman to make observations of particular value. Little diminution in sunlight is perceived until more than half the sun's disc is covered by the moon. An appreciable fall of temperature occurs during a large partial eclipse of the sun.

Solar Eclipse. A total eclipse of the sun is perhaps the grandest of all natural phenomena. While almost of annual occurrence, its visibility on any occasion is confined to a very small area, along a line usually less than 100 n. mile wide, so that in any fixed place it is in general very rare. The duration of the total phase is very short, usually from a few seconds up to about two minutes, though in very exceptional circumstances it may be considerably more, the possible maxima being nearly eight minutes. During totality the fall of temperature is marked; often the wind changes or springs up, if previously calm. The sky darkens and has a peculiar appearance, often with lurid cloud colours. During totality the bright planets and the brighter stars may be seen.

Very occasionally a ship at sea or in harbour may be on the line of totality and several of such observations have been received in the last 50 years. The seaman fortunate enough to witness such an eclipse should endeavour to record all that he sees in as full detail as possible. There is so much to see in such a short time that it is desirable for several persons to observe in company. At the instant the moon finally covers the round body of the sun normally seen, the solar corona will spring into view. This is an irregularly extended atmosphere of the sun, pearly-white in colour, giving about half as much light as the full moon. It has a definite shape which varies according to the position of the year of observation in the 11–year cycle of solar activity (see under Sunspots).

Near the time of maximum activity the corona is disposed fairly equally round the sun, with a definite structure of rays and bands, and sometimes curved forms like flower petals. Near the time of minimum activity the corona shows much less structural detail and the form is quite different. A wide band, more or less parallel sided, stretches outward from the equatorial region of the sun, one on each side of the sun, and these bands may extend a long distance, up to two or more solar diameters. At this time the polar regions usually show only a few short rays of coronal light. In the intermediate years of the solar cycle, the corona assumes forms intermediate between those described above.

Owing to the short duration of total solar eclipses and their comparative rarity, the total time for which the corona has been seen in the last 150 years is probably about two hours. Its exact form on any particular occasion is unpredictable. Marine observers can therefore make observations of real scientific value if the form, extent and detail of the corona is carefully noted and sketched. As the fainter extensions of the corona are best seen with the unaided eye and the structural detail is best seen with binoculars or a small telescope, it is best, especially when the duration of totality is short, to have two observers, each working in one of these different ways.

One or more of the great rose-red eruptions of hydrogen and calcium gas from the sun, known as prominences, may be seen adjacent to the moon's limb without optical assistance, especially if the sun is near its state of maximum activity. Unlike the corona, these may be seen in full sunlight on any day, by astronomers using special apparatus. Other features of a total eclipse on which attention may be concentrated are (a) meteorological effects (b) the changing colour effects of sky and cloud and the rapid onrush of the moon's shadow through the air as the total phase begins, (c) the visibility of planets and stars.

World Solar Eclipses. For information on the schedule of expected solar eclipses, both total and annular up to the year 2015, see below

WORLD SOLAR ECLIPSES*

For the information of mariners, a schedule of expected solar eclipses, both total and annular up to the year 2015, follows.

Two eclipses during the period will be visible from parts of the United Kingdom:

1. The total eclipse of 11 August 1999, visible from the Scilly Isles, Cornwall and Alderney. 2.

The annular eclipse of 31 May 2003, visible from northern Scotland.

MaximumMaximum

Type' Date Duration 2 Lat 3 Long 3 Location

A 29.4.1995 06 37 4º S 79º W South Pacific, Ecuador, Peru, Brazil

T 24.10.1995 02.11 8º N 113º E India, Thailand, Cambodia, Sabah

T 9.3.1997 02.50 57º N 113º E Mongolia, Russia, North Polar

T 26.2.1998 04.09 4º N 82º W Galapagos, Colombia. Venezuela,

Antigua, Montserrat

A 22.8.1998 03.14 3ºS 145ºE Sumatera, Sarawak. Melanesia, Pacific

A 16.2.1999 00.4. 39º S 93ºE South Indian Ocean, Australia

T 11.8.1999 02.23 45º N 24º E North Atlantic, Scillies. Cornwall, Europe, Turkey, Iran, India

T 21.6.2001 04.57 11º S 3º E South Atlantic, Southern Africa

A 14.12.2001 03.53 1º N 130º W Pacific, ends in Costa Rica

A 10.6.2002 00.23 34º N 178º W Pacific in entirety

T 4.12.2002 02.04 39º S 60º E Southern Africa, South Indian Ocean

A 31.5.2003 03.37 67º N 24º W Scotland, Iceland, Greenland

T 23.11.2003 01.57 72º S 88º E Antarctica in entirety

A/T 8.4.2005 00.42 10º S 119º W Pacific. ends in Colombia

A 3.10.2005 04.31 13º N 28º E North Atlantic, Spain, Africa, Indian Ocean

T 29.3.2006 04.07 23º N 16º E Atlantic, Africa, Turkey, Black Sea. Russia

A 22.9.2006 07.09 21º S 9º W Atlantic in entirety

A 7.2.2008 02.12 67º S 150º W Antarctica, South Pacific

T 1.8.2008 02.27 65º N 72º E Greenland, North Polar, Russia, Mongolia, China

A 26.1.2009 07.54 34º S 70º E Indian Ocean, ends in Sumatera

T 22.7.2009 06.39 24º N 144º E China, Pacific

A 15.1.2010 11.08 2º N 69º E Ethiopia, Indian Ocean, India, Sri Lanka, China

T 11.7.2010 05.20 20º N 122º W South Pacific, ends in Chile

A 20.5.2012 05.46 49º N 176º W Japan, North Pacific, California

T 13.11.2012 04.02 40º S 161º W Queensland, Pacific in entirety

A 10.5.2013 06.04 2º N 175 º E Australia, Solomon Islands. Pacific

A/T 3.11.2013 01.40 4º N 12º W Atlantic, Central Africa

A 29.4.2014 00.00 70º S 131º E Antarctica in entirety

T 20.3.2015 02.47 64º N 6º W North Atlantic, Faeroes, North Polar

Notes:

I. T – Total

A/T – Annular/Total. Starts and ends as annular, total in the middle.

A – Annular.

2. Duration of totality or annularity in minutes and seconds of time.

3. Location of maximum eclipse.

*Information (September 1994) by courtesy of Mr Sheridan Williams, The Clock Tower. Stockgrove Park, Leighton Buzzard. Bedfordshire LU7 OBG. United Kingdom, to whom any queries or requests for further information should be addressed.

Lunar Eclipse. The total phase of a lunar eclipse generally lasts a considerable time, sometimes for nearly two hours; the exact duration depends on how centrally the moon passes through the Earth's shadow. The totally eclipsed moon usually remains visible, appearing in some shade of red or copper. Careful observation of this colour, and its changes, if any, during the total phase are of value. A general statement of the degree of brightness of the totally eclipsed moon should also be given, noting how far its surface markings remain visible. The totally eclipsed moon receives reddish sunlight by refraction through the section of the Earth's atmosphere in profile to the moon at the time, and the amount and colour of the refracted light vary according to the cloudiness and other meteorological conditions in this part of the atmosphere. When fine dust in sufficient quantity is suspended in the air after a big volcanic eruption, the moon may almost, or even completely, disappear from sight during total eclipse. Such an observation should be carefully recorded, with all relevant detail.

COMETS

Comets are members of the Solar System, moving in elliptical orbits, in most cases so enormously elongated that the period of revolution round the sun may be hundreds or even thousands of years. A few return in a comparatively short time one of these being the well- known Halley's Comet, with a period of about 76 years, last seen in 1985/6.

Comets are much less dense than planets, and consist of a loose aggregation of widely separated small solid bodies, ranging from the size of a grain of sand to that of small stones, probably with an admixture of larger pieces. The diameter of this collection is usually only a few hundred miles, but may be several thousand. Comets are only seen in that part of their orbit near the sun, when they shine partly by reflected light but mainly by the vaporizing of the material of the comet by the sun's heat. An interesting feature of a comet is its tail, which is only formed when the comet is relatively near the sun. This consists of dust and gases ejected from the head, probably by light pressure and electrical repulsion. The tail of a large comet may be many millions of miles in actual length. The apparent length may be anything from a degree or two to 60º or 80º or more. The direction of the tail is from the comet's head away from the sun. This direction bears no relation to the direction of the movement of the head of the comet in its orbit. The tail of a comet, unlike the transitory trail of a meteor, therefore does not show the direction in which the comet has travelled.

Most comets never become bright enough to be seen without telescopic aid and some never develop tails, but a bright comet is a magnificent naked-eye spectacle. There should be no confusion between the appearance of a comet and a meteor. A meteor is only seen for a few seconds as it travels more or less rapidly over its apparent path in the sky. A comet remains apparently fixed among the stars and sets with them in due course. It has a continuous movement relative to the stars, but in most cases this can only be seen in a naked-eye or binocular observation by comparing its position on successive nights. The period of naked-eye visibility of a comet may be anything from a few days to a number of weeks. It finally becomes invisible by either getting too faint, or passing into the daylight region of the sky or changing in declination so as to sink below the horizon.

Astronomers measure the position of the head of a comet relative to stars near it in the field of view of a telescope, or large-scale photographs may be taken. From a minimum of three such observations on successive nights, the comet's orbit in space and its subsequent apparent track in the sky can be computed. Angular distances of the comet from two or three bright stars, measured by sextant, are not sufficiently accurate for this purpose, but serve to identify the object and help in making an accurate sketch of the comet and its tail in relation to the stars. It may occasionally happen that more than one naked-eye comet is visible at the same time.

Valuable observations of a naked-eye comet may be made at sea, and it may happen that some interesting feature is seen which would not otherwise be put on record, if conditions of daylight or cloud make observations impossible in other parts of the world at that particular time. The brightness of the head and the form and length of the tail may sometimes change appreciably from night to night. The brightness of the head is estimated by comparison with that of neighbouring stars or planets, as described under Novae below. The altitude of the comet's head should be given, as part of this observation, also notes on the state of the sky, such as whether thin cloud, haze, twilight or moonlight is present. Careful sketches of the form and length of the tail are valuable and should include details of the structure of the tail, if any are seen, stating whether the observation was made with the unaided eye, or with binoculars. The end of the tail usually fades very gradually into the dark sky and the method of averted vision can be used to see it as far as possible; binoculars will not show the fainter extension. It is of special importance to record any tails, other than the main one, which may be visible; these are normally on the same side of the head as the main tail, making various angles with it, and they are usually narrower and fainter than the main tail. On rare occasions a short tail pointing towards the sun may be seen, i.e. in a direction opposite to that of the main tail. If the comet shows any peculiarity of colour this should be noted.

THE ZODIACAL LIGHT AND ASSOCIATED PHENOMENA

The zodiacal light

This is observed as the cone-shaped extremity of an elongated ellipse of soft whitish light which extends from the sun as centre, extending above the westerly horizon in the evening or the easterly horizon in the morning. The best time for observation is just after the last traces of twilight have disappeared in the evening, or just before the first traces appear in the morning. The light retains its apparent place among the stars and gradually sets or rises with them. It is more brilliant in the tropics, but is very conspicuous even in temperate latitudes, if observed away from the glare of large towns.

The axis of the light lies in the zodiac, very nearly but not quite in the plane of the ecliptic. In tropical latitudes, where the ecliptic makes a large angle with the horizon at all times of the year, the light may be seen well on any clear night or morning in all months. In the temperate latitudes of the Northern Hemisphere it is best seen in the evenings of January to March and in the mornings of September to November.

The light is pearly and homogeneous and differs markedly in quality from that of the Milky Way, the brightest part of which it may considerably exceed in luminosity. Its luminosity decreases with altitude above the horizon, since its brightness is greater the nearer the observed point is to the sun's position below the horizon. It appears, however, to fall off in brightness near the horizon on account of the greater thickness of the atmosphere its light has to traverse. At any altitude the axis of the light is brighter than its lateral parts. In northern temperate latitudes the edge of the cone towards the north in azimuth is less well-defined than that towards the south and tends to spread northwards near the horizon.

The zodiacal light is believed to be a cosmic phenomenon, due to the reflection of the sun's light from dust or gaseous matter, extending outwards to a point somewhat beyond the Earth's orbit. There is much that is not known about this phenomenon and new observations from all latitudes will be of real value. Any features of interest should be noted, such as the colour of the light and any irregularity of form or light distribution. Observations of its brightness will be of value, as it is not yet known whether this is constant on successive nights or in different years. Apparent changes of brightness often occur since the night sky is not always equally transparent. The presence of a bright planet, especially Venus, in the region of the light dims it considerably. Estimates of brightness should be made on moonless nights, after all twilight has disappeared, by comparing the light with that of the Milky Way, preferably at about the same altitude. The position of the Milky Way should be specified, as this varies markedly in brightness in different parts of the sky. Thus the light on a given night might be estimated to be twice as bright as the Milky Way in Cygnus.

Observations of the precise position of the light, about which there is still some uncertainty, may be made by a careful sketch of the cone showing the position of specified stars, either within, on the edge of, or outside the cone.

Zodiacal band and Gegenschein. Joining the apexes of the cones of the morning and evening zodiacal lights is an extremely faint luminous band, a few degrees wide, lying along or nearly along the ecliptic, called the zodiacal band. On this band, at a point very nearly or exactly 180º from the sun's position in the ecliptic, is a somewhat brighter and larger but ill- defined patch, 10º or more in diameter, known by the German name 'Gegenschein'. This therefore is due south (in the Northern Hemisphere) at midnight, local time. These phenomena may be observed in temperate latitudes on the clearest moonless nights when at sufficient altitude; they are somewhat brighter in the tropics, on account of the greater altitude of the ecliptic. Further observations of these phenomena are much desired, especially from tropical localities. The track and width of the band, and the size, shape and position of the Gegenschein should be noted, together with variations of brilliancy and any special features seen, but the observation will be found difficult even to keen eyesight. The Gegenschein is usually invisible for the few nights on which it is projected upon the Milky Way in its annual journey round the ecliptic.

NOVAE

Sometimes, quite unpredictably, a small star, usually such that a telescope is required to sight it, brightens up very much, within a few hours or a day or two at the most. This is, somewhat loosely, called a 'nova' or 'new star'. While many of these never become visible to the naked eye, occasionally one does so and may even reach the first magnitude, or brighter, thus completely changing the aspect of the constellation in which it appears. If conspicuous, a nova is generally mentioned in the newspapers. Should the marine observer hear of one, or discover one (in which case he will usually find he is not the first discoverer) he may be interested in following its changes of brightness. The normal history of a nova is that it remains at full brightness for a short time, probably a day or two at the most, and then very gradually decreases, the reduction in brightness being interrupted by slight temporary increases. If the star has attained the third magnitude or more it may remain visible to the naked eye for several weeks.

If the observer wishes to record the exact brightness of a nova (or other star) at any time, he may select a star of about the same altitude judged to be exactly of the same brightness. If no such star is available, he should select two stars of about the same altitude as the nova, one a little brighter and one a little fainter than the nova. He can then express the brightness of the nova in terms of the small interval of brightness between the two comparison stars. For example, it might be halfway between them in brightness, or one-third of the interval, counting from the brighter to the fainter, or one-quarter of the interval, counting from the fainter to the brighter. If such an observation is received, it can be easily converted into actual magnitudes, since the magnitudes of all naked-eye stars have been accurately determined. Both these methods break down if the star is much above the first magnitude, as suitable comparison stars would probably not be available. One or more of the bright planets, if visible, might, however, serve for this purpose.

An accurate observation of the magnitude of a nova, especially in its early stages when the brightness is changing quickly, may be of great value to astronomers, since no other observation might have been made anywhere else at the same time.

SUNSPOTS

It is very dangerous to the sight to look at the sun, either with or without optical aid, without using smoked or deeply tinted glass to reduce the light. This applies even when the sun is in partial eclipse. The only exception is when the sunlight is greatly weakened by passage through fairly thick fog, especially when the sun is at low altitude.

The number and size of sunspots varies in different years. Over a period of years solar activity, of which the occurrence of large sunspots is one manifestation, rises to a maximum and subsequently falls to a minimum. The time between successive maxima varies considerably, but averages about 11 years. For several years around the time of maximum activity, spots are frequently large enough to be seen without optical aid; sometimes two or more are thus visible at the same time. Around the time of minimum activity, spots are either very small or completely absent. The life of an individual spot may be anything from a few days to several weeks.

Owing to the sun's rotation on its axis, a spot previously formed, and coming into view at the sun's eastern limb, will appear to cross the disc in about 14 days, if it lasts so long. Apparent changes of position of the spots on the sun's disc take place during the day, but are merely due to the observer's changing angle of view. The imaginary line forming the horizontal diameter of the sun at noon appears to be tilted upward between sunrise and noon and downward between noon and sunset, the most extreme tilting occurring at sunrise and sunset.

Daily photographs of the sun through telescopes are taken at one or other of the astronomical observatories throughout the world. While marine observers may find it interesting to see the spots and note their changes of form and position on successive days, especially in years of maximum solar activity, it is not necessary to make sketches of them in the logbook as these can never be accurate enough to have any scientific value.

Solar flares. Near certain sunspots there occur areas which undergo sudden increases in brightness; these are called flares. They are best seen by means of special instruments which give a picture of the sun's surface in red hydrogen light. Some of the greatest solar flares have, however, been observed as increases in the total white light of the sun; seen in this way, a flare lasts for a few minutes and has about the same area as a large sunspot. The first such observation of the bright patch on the sun's surface was made in 1859 and several flares have been similarly observed since then. The appearance of flares cannot be predicted, but they are more numerous at times of maximum solar activity (as measured by the numbers of sunspots).

The increase in light intensity during a flare is particularly strong in the ultraviolet part of the spectrum (the part beyond the visible violet light to which our eyes are not sensitive). The blast of ultraviolet light emitted from a solar flare produces several detectable effects in the high atmosphere of the Earth.

Associated with the increase in light intensity during a flare, there is ejection of material particles from the region of the flare out into space. This material shoots out at speeds of about 300 to 650 kilometres per second, which probably increases as the material gets further from the sun. If moving in the appropriate direction, this material causes interesting effects in the high atmosphere. Some of the high atmospheric effects of solar flares are described in the next chapter.