March, 1902.] 



KNOWLEDGE. 



59 



higher, so the glow in the cast beconifs brighter, ami the 

 fainter stars die out, until at length all have disapiK'ared 

 in the increasing daylight and the sun itself arises, the 

 bright herald which preceded it being the only one of all 

 the starry company that remains visible till the sun's 

 appearance. 



The planet Venus is the only object that in this coni- 

 pletest manner can act as the forerunner of the sun, and it 

 rightly, at such seasons, has a spcci;d claim to the title of 

 the " Morning Star." But it is only now and then that 

 Venus is so placed as thus to act as the solar herald. At 

 other times the role has to be filled by stars of a meaner 

 rank. But it is well worth noting what stars are the last 

 to rise before the dawn drowns them with its growing 

 light. The next morning it will be found that those same 

 stars are visible just a little longer, and the next morning 

 longer still, and so on, until some other star shows itself 

 as the one to climb up from the eastern horizon just before 

 the opening daylight becomes strong enough to overcome 

 its shining. 



Such stars are each in their turn " morning stars," and 

 their first appearance in the faint daven-glow of the east 

 before sunrise is the " heliacal rising " which was made 

 much of by ancient astronomers and poets. Not without 

 reason, for it is an observation of very considerable exact- 

 ness, and one which requires absolutely no instrument ; 

 not even the simple one of an obelisk or an upright spear, 

 still less of the solid masonry of a " solstitial " or 

 " equinoctial temple,' ' or the huge trilitbous of a Stonehenge. 

 And it fixes the return of the sun to the same part of the 

 heavens at the end of a year quite as exactly as those more 

 cumbrous conti-ivances could do, if indeed they ever were 

 used for such a purpose. 



Corresponding to the " morning stars " are the " evening 

 stars," the stars wliich are seen in the western twilight 

 just before they set; the gathering darkness penuitting 

 them to be just glimpsed for a minute or two before they 

 follow the departing sun down to the under world. This 

 constitutes their " heliacal setting," and its observation 

 supplements that of the "heliacal rising." 



All stars are not by any means equally suitable for 

 observation of theii heliacal risings and settings. It is 

 of course clear to begin with that the circumpolar stars, 

 the stars wliich never set, are quite useless in such a 

 connection. Nor are the stars in the extreme south, which 

 only rise at the best a few degrees above the horizon, and 

 only remain visible a very few hours, well suited for the 

 work. The best stars are those which are not situated at 

 any very great distance from the equator, either north 

 or south, but which lie between the tropics of Cancer and 

 Capricorn. Kigel, Sirius, Procyou, Arcturus, Alpha Ser- 

 pentis. Alpha Ophiuchi are fairly well placed for observa- 

 tion in England, both at rising and setting, but as a 

 general rule we may take it that the time of the spring 

 equinox is the best for observing heliacal settings, and 

 that of the autumnal for observing heliacal risiugs. The 

 reason of this will be seen at once from the accompanying 

 diagram, which represents the effect of the sun's motion 

 in declination at setting, at the spring equinox ; at the 

 autumnal equinox the conditions are, of course, exactly 

 reversed. 



K the sun moved always in the equator, then a star 

 which rose on one day at precisely the same time as the 

 sun would rise about 4 minutes earlier than the sun on 

 the next, and so en day by day. But at the time of the 

 spring equinox, the sun is moving northwards at the rate 

 of about 24 minutes of declination every day. Conse- 

 quently the sun rises about 2j minutes earlier every day, 

 and the stars which rise at dawn only gain therefore upon 

 the sun 4 m. — 2 J m. ; that is to say, 1 j minutes. If the 



star is north of the equator, the sun is also coming nearer 

 to it day by day, aud hence that region of the sky becomes 

 brighter and brighter at dawn. The result is that there 

 may readily l)e a good deal of uncertainty about the 

 determination of the day of the heliacal rising of such a 

 star under these conditions. For the setting, the condi- 





Positions of tlio Suu below the East horiion, ucai' tlie Spring 

 Equinox, at the rising of A Piscium. 



tious are reversed. If the sun were always in the equator, 

 a star which set with the sun one day would set 4 minutes 

 before it the next, aud so on day by day. But since the 

 sun is moving northward at the spring equinox, it sets 

 about I-3 minutes later each day, aud the star therefore 

 would set before the sun 4 m. + 1| m., or Sf minutes the 



Positions of the Sun below the West horizon, near the 

 Spring Equinox, at the setting of i Piscium. 



next night, aud so on night after night. If the star was 

 south of the equator, the sun would be moving away from 

 it in declination, aud under these circumstances we should 

 get the sharpest determination of the date of heliacal 

 setting. So the autumnal equinox gives the best oppor- 

 tunity for determining heliacal risings, specially for stars 



