GLORY OF THE STARS.

New Zealand Herald, Volume LXIV, Issue 19730, 1 September 1927, Page 13

GLORY OF THE STARS.

THE HEAVENS IN SEPTEMBER. PLANETS AND CONSTELLATIONS. BY CRUX AUSTRALIS. The Sun is in the constellation Leo until September 18, when it passes into Virgo. It is moving south in the sky, increasing its altitude at noon at Auckland from 44£ deg. on the first to 56 cleg, on the 31st. On the 24th the Sun crosses the celestial equator, and our spring equinox occurs. During the earlier part of last month a splendid naked-eye sunspot group was visible, passing off the Sun's limb about August 20. Naked-eye spots have been fairly rare this year, but, as the time of maximum spottcdness is approaching, many large groups may bo expected in the next few years. The Moon will be in conjunction with Saturn on the morning of the 4th, Jupiter on the evening of the 12th, Venus on the morning of the 24th, Mars on the night of the 26th, and Mercury on the evening of the 27th. On the 3rd Mercury is in superior conjunction with the Sun and enters our evening sky. Its path this month is in Loo until the 10th, when it passes into Virgo. On the 16th it will overtake the more slowly-moving Mars, being one-tenth of a degree south of Mars at 5.30 p.m. on that day. The two planets will be rather too near to the Sun to be observed with the naked eye. Venus, which has been shining so brightly in our evening sky for some months past, will be in inferior conjunction with the Sun on the 11th, then passing into the morning sky. Its motion will be retrograde until the end of the month, and its path lies within the constellation Leo. Mars is now very close to the Sun, with which it will be in conjunction next month, and is not a prominent object. Its path is confined to the constellation Virgo during September. Jupiter is now very favourably situated for observation, rising about 7.30 p.m. at the beginning of the month, and earlier on each succeeding evening. It will be in opposition on September 22, when it will bo in that part of the heavens directly opposite the Sun, and will be visible throughout the night 1 in the constellation Pisces. The most prominent features visible on Jupiter are the dusky belts always running parallel to the planet's equator, but subject to changes in their form and visibility. Now that the planet is so favourably placed, the possessor of a small telescope will find much to interest him by studying these objects. Besides the beauty of the belts, even a small telescope will show the interesting phenomena connected with Jupiter's four brightest satellites as they travel round their primary. Transits, eclipses and occultations of its principal moons will be j visible on September 2, 3, 5-10, 20-25, 27-30. Saturn still lies in the constellation Scorpio, being almost overhead in the early evening at the beginning of the month. Its wonderful ring system is a source of never-failing interest to those who possess telescopes. Places of the Constellations. The constellations may be seen in the positions given below, at 10 p.m. of the Ist, 9 p.m. of the 15th, and 8 p.m. of the 30th of the month. Lyra is now setting in the north-west. Vega, one ol the brightest stars in the northern hemisphere, is a member of this group. Just above the horizon in the north is the splendid cruciform group of Cygnus. A faint member of this group, called 61 Cygni, was the first star to have its parallax determined. It is a double star, the two components giving together only about one-tenth the light of the Sun. Above Cygnus are the small groups of Vulpecula and Sagitta, and still higher on the meridian, in the flow of the Galaxy, is Aquila, whose brightest star is Altair. Andromeda is rising in the north-east, with the large group of Pegasus above it. Above Pegasus lie Aquarius and Capricornus, the latter being slightly north of overhead. Its brightest member is a pair easily separable to the naked "eye, and each of these is accompanied by several fainter stars, the whole forming an interesting telescopic group. Pisces and Cetus are rising in the east and south-east respectively, and above Cetus is Pisces Australis, containing the brilliant Fomalhaut The bright Achernar of Eridanus is now well up in the south-east, and above it and near the pole are the small groups of Phoenix, Grus, Lndus, and Hydros. Under the pole lies Dorado, another small group, while the Cross and Centaurus are near the horizon in the south-west. Libra and part of Serpens are setting in the west, while above these are Scorpio and Ophiuchus. The remainder of Serpens lies between tho latter and Sagittarius, now almost overhead. Close to the south of Sagittarius is the asterism Corona Australis, and between Scorpio and the Cross are Lupus and Ara. A Wonderful Variable Star. Algol, in the constellation Perseus, shines for most of the time with a steady light as one of the brightest members of the group, but at regular intervals of 2 days 20 hours 48 minutes it fades rapidly in brightness, remains thus for a short time, and then recovers its light. The Arabs called this star el Ghoul, or "the demon," which seems to indicate that they were acquainted with its remarkable behaviour. Very soon after its law of variation was found by Goodricke, a deaf and dumb astronomer, in 1782, it was conjectuied that its variability was caused by a dark body revolving round the bright star and periodically cutting off. part of its light in eclipses. Vogel, in 1888, by means of the spectroscope, was able to prove that this conjecture was correct, and that Algol was really a binary star, the plane of whose orbit lay almost in the line of sight, thus making the periodic eclipses possible It was possible then to determine the orbit, of Algol and to estimate the sizes of the two bodies Algol itself has 1.45 times the radius of the Sun and its mass is 0.37; the radius of the dark companion is 1.66 times that of the Sun and its mass 0.18. The distance between the centres of the two bodies is only 4.77 times the radius of Algol and the mean density of the system only 0.07 that of the Sun. By the application of the selenium cell, which is extomelv sensitive to variations of light, it was found that a very small secondary eclipse occurs when the satellite is passing behind Algol, and it, is therefore not absolutelv dark, but possesses a faint light of its own. From the amount of light lost, during the principal eclipse it is found that at that time two-thirds of Algol's surface is obscured by the fainter companion. 'The times when Algol undergoes eclipse are predicted each yenr in the British Astronomical Association Handbook, and four eclipses occurring this month will be visible in New Zealand. These will occur on September 3. about 6.36 a.m., September 6, about 3.30 a.m., September 26, about 5.06 a.m., and September 29, about 1.54 a.m The times given above are the approximate times of mid-eclipse; the whole eclipse, however, lasts a little over eight hours. Four hours before mideclipse the star's light suddenlv begins to I diminish until" mid-eclipse. _ wben. without. any appreciable pause, it rises atrain to its normal brierhtness. The constellation Perseus, in which this variable star is placed, rises on September 1 about 1 a.m., an hour earlier on the 15th. and two hours earlier by the 30th, Tn these latitudes it never rises much above the northern horizon, bnt. Algol can be easily found by those who wish to observe its eclipses. At an altitude of about 25deg. at 3 a.m. on September 1, the Pleiades, the "seven sisters," are a prominent group. Belcw them and a little to the west are two moderately bright stars, the lower and brighter one being Algol.