ASTRONOMICAL NOTES

Evening Star, Issue 19803, 29 February 1928, Page 2

ASTRONOMICAL NOTES

THE SKYS IN MARCH [Written by A. G. Crust, M.Sc., for the ‘ Evening Star.’] On March 2, at 10 p.m., New Zealand summer time, the Moon is to be found south of the star Pollux, in Gemini. On March *l, at 3 a.m. summer time comes to an end, the clocks will be put back one hour, and we shall be again only llh oilman ahead of Greenwich mean time. The rest of these phenomena arc, therefore, given in New Zealand standard time. On the sth the Moon passes north of Neptune at 0.30 a.m. and Regulus at 12 noon; on the 6th full moon occurs at 11 p.m.; on the 9th, at 7 p.m., the iMoon passes Spica; on the 13th, at 7 p.m., Antares; and on the 14th, at 5.30 p.m., the planet Saturn. At 3 a.m. on the 15th the Moon is at last (punter; at 0.30 a.m. on the 19th she is south of Mars; on the 20th, at 4.30 a.m., south of Mercury, and at 5.30 south of Venus. New moon occurs at 8 a.m. on the 23nd; Uranus is passed at 8.30; Jupiter on the 23rd, at 0.30 a.m.; Aldebaran on the 26th, at 11 p.m.; and Pollux on the 30th, at 3 a.m.

Mercury, which is in the morning sky, is stationary on the 9th, in conjunction with Venus on the 18th, at 5.30 a.m., and at greatest elongation west on the 23rd. On the JBth Mercury will bo north of or below Venus.

Jupiter is now drawing near the sun, and sets about 7 p.m. Saturn is in ipiadniture, or ilddeg lYom the Sun’s position in the ecliptic, on the U)th, at 2.30 a.m., rises about 9 p.m., on the 15th, and is stationary on the 29th.

Uranus is badly placed for observation, but Neptune is in a favorable position, being near tho bright star Regulus, Alpha Lconis. On tho loth tho Sun rises at 6.30 a.m. and sets at 6.31 p.m. The autumn equinox occurs on tho 21st, at 8.12 a.m.

On tho loth tho great star Canopus is on the meridian overhead at 7 p.m.; Sirius north, but high up, at 7.10; Procyon north at 8.13; and Regulus north at 10.41 p.m. Orion is now west of the meridian in the early evening, but still may be observed to advantage. Canopus is the first important star ot the larger constellation Argo, which is high in tho south-east. Argo contains many bright stars, and the variables 1 Canine and L2 Puppis, of the Cophcid type; also the Algolicl V Puppis. Sirius is in Canis Major, Procyon in Canis Minor, these groups representing Orion’s two dogs. Regulus is tho chief star of Leo, tho Lion, between Leo and Gemini is Cancer, with the hazy cluster of stars called the Pnosopo, or Beehive. Above Cancer and east of the little dog is tho head of Hydra, containing the remarkable binary Epsilon Hydra;. Local sidereal lime, .08h, Lat. 46dog. S.

The following slur positions hold good at Dunedin for March 9 at S) p.m., March 2d at S p.m., and April 9, at 7 p.m.:—

Itegulns, N.N.E. by K, altitude 26dcg; Spica, E., lodeg; Alpha Gmcis, S.E., 50deg; Alpha Centanri, S.S.E. by E., Jsdcg; Beta Argus, S.S.E. by S., (jtklcg; Alpha Grids,. S.S.AV., Tdog; Achcrnar, S.W., 34dcg; Alpha Pluenicis, S.AV., Itideg; Cajiopus, W.S.W., alt. Tlideg; TJicta Eridani, W.S.W., alt. t!7dog; Tfigel, W.N.AV. by N., 37dog; Aldobaran, N.W., 12dcg; Botclguesc, N.N.W. by AV., 29deg; Sirius, N.N.AV. by AV., 57dog; Procyon, N.N.AAk, 39deg; and Pollux, N.N.AAG by N., altitude Kideg. On March 9, at 9 p.m., the Moon is past the full, and is visible nearly due oast, 9deg above the horizon. She is not visible on the 24th, at 8, or on April 9, at 7 p'.in. s None of the bright planets is visible at the above-mentioned times. Observers anywhere in New Zealand should find these positions sufficiently accurate lor identifying any of these bright stars. 'The differences of longitude, except in the case of Napier or Gisborne, are barely sufficient to alter the azimuths by one point of the compass. The effects on altitudes duo to differences in latitude arc more serious. Auckland observers will find northerly stars lOdeg higher, southerly ones lOdcg lower, than the values here given. The differences oast and west will be very small. . , .. In ancient times astronomers behoved that the earth upon which they stood was quite flat, except for occasional mountain ranges, and that the sea, which surrounded the known world, stretched in a level expanse to inconceivable distances. AVhilo the ancients lived on a real globe and believed it flat, they imagined also that the great vault which they saw above them day and night was a. real, solid, concave surface, upon which tho stars were immovably lixed. Thus they believed that the flat earth stood still, while the the vast .sphere of the sky rotated around it daily, carrying tho sun, moon, and planets, and the immovable Although we now know that the earth is a rotating sphere and that the stars are nob fixed upon any surface, wo still use astronomcial terms and measurements which have come down to us from those ancient times, and ninny of these terms are so commonly found in astronomical literature that it is important to know their meanings. Several methods ol measuring tie sky-sphere around us have been devised, but all are similar in principle ami closelv intcr-relalcd. I? we arc sailing mi'the open sea, with no laud in sight, wc see what appears to he a straight lino where the sky meets the sea;“but is it a straight line? Jt appears in whatever direction wo look, and at the same distance in every direction, so that it is really a circle at whose centre we, ourselves arc situated. '1 his circle f- called the horizon, ami heavenly' bodies can ho seen only when (hey arc above tills horizon. The'very highest point of the vault above us is called tho zenith, while the point directly under our feet is_ called the nadir. Surveyors and navigators frequently make measurements of terrestrial and celestial objects, using the horizon and the zenith _ as starting points. All these spherical measurements are expressed as angles, and an angle may he most simply defined as the difference in tho directions of two points ns seen ii;orn a third. r iluis the amdo between tho zenith and the horizon is 90deg, while an o>ect halfway between the horizon and tho zenith is said to have an “ altitude ” of 45deg. A mountain rising to one-third of the distance from horizon to zenith has an altitude of SOdeg, but wo cannot know its height in feet unless we know the distance of its summit iu feet. Angles measured around the horizon are called “ azimuths,” and are generally measured from the north point right around the compass, although nautical men usually call such angles “ bearings,” and measure them from north or south towards east or west. Thus the direction south-east is read as 90 plus 45, equals 135 deg of azimuth, or as a “ bearing” 45deg east of south. Usually measurements of azimuth are begun from the north, and a line drawn from the north point across the sphere to the zenith, then down from the zenith to the south point, is called the meridian. The word meridian comes from the Latin “ineridies” (midday), because the sun is on this north-zenith-,south line at the middle of tho solar dav. A similar line running vertically from the east point to the zenith and down to the west is callcd the “prime vertical,” and is a convenient starting line for certain calculations. If we were able to go to the South Pole we should find that all the stars revolved around the zenith or overhead

point, while not one ever rose or set throughout the twenty-four hours, or. indeed, throughout the six months ol winter night. The point which is the zenith at“thc South Pole is practically fixed among the stars, and is known as the South Celestial Pole, while the circle which coincides with the horizon for a. nolar observer is halfway between the North and South poles, and is therefore in the plane of the-Equator. At the South P.olo there is no meridian, prime vertical, or azimuth. Strictly “south” is “up” and “north” is “down”; “west” is a turning from right-to left, as the stars go, and to go “ east ” you turn to the right. Measurements of altitude at the South Pole are also measurements of distance from the Equator of the heavens, and such distances from the Celestial Equator arc called “ declinations.” Tims declinations in the heavens correspond with “latitudes” on the spherical surface of the earth. Let us now imagine ourselves on the Equator, in some region with an unbroken horizon. The actual North and South Poles of the heavens lie duo north and south, exactly on the horizon, while the meridian passing overhead cuts at right angles the prime vertical, which is identical here with the Eouator of the heavens, just as our geographical east-west line coincides with the earthly Equal or. If wo could wait lor twentv-four hours and have the sun convenientlv removed we should be able to see all the stars in the heavens Iroin this enualorinl station. The order in which the stars rise in the east here is the order which gives us the measurement corresponding to longitude upon the earth, and this measure is termed “ right ascension,” because it gives us the times of “ascension ” of the stars from the horizon in this “right” or ideal locality on the Equator. We also may speak of the “ hour angle ” of a star, which is its distance from tho meridian, measured along the Equator, and which is. of course, always rapidly changing. Eight ascensions are measured from tho point in tho' heavens whore the sun crossed the Equator in the direction from south to north, as it does on the 21st of this month, and are usually expressed in hours, minutes, and seconds of sidereal time. ( Tho sidereal time at any place is then the same as the rierht ascension of the stars noon its meridian. The right ascensions of all stars change slowly from vear to year, because of a westerly movement along the ecliptic or solar path of the point where the sun crosses the Equator. This point is called tho First Point of Aries, but it i_s now situated in the constellation Pisces. Its westerly movement is termed .the Precession'of th Equinoxes, and causes a progressive increase in the right ascensions of most stars, which are also subject to minor changes, predicted for the brighter stars in the nautical almanacs published by various nations. If we remember that the declinations are also affected by these changes and that the stars have, their own individual motions in the heavens we see that they are far from being as immovable as the ancients imagined.