Epoch J2000.0 Equinox J2000.0
|Right ascension||08h 44m 41.09921s|
|Declination||+18° 09′ 15.5034″|
|Apparent magnitude (V)||+3.94|
|Spectral type||K0 III|
|U−B color index||+0.99|
|B−V color index||+1.08|
|Radial velocity (Rv)||±0.2516.39 km/s|
|Proper motion (μ)||RA: −17.67 mas/yr
Dec.: −229.26 mas/yr
|Parallax (π)||24.98 ± 0.24 mas|
|Distance||131 ± 1 ly
(40.0 ± 0.4 pc)
|Absolute magnitude (MV)||+0.843|
|Surface gravity (log g)||2.7 cgs|
|Metallicity [Fe/H]||−0.13 dex|
|Rotational velocity (v sin i)||2.8 km/s|
It bore the traditional name Asellus Australis which is Latin for "southern donkey colt". In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Asellus Australis for this star on 6 November 2016 and it is now so entered in the IAU Catalog of Star Names. Together with γ Cancri, it formed the Aselli, flanking Praesepe.
In Chinese, 鬼宿 (Guǐ Su), meaning Ghost, refers to an asterism consisting of Delta Cancri, Theta Cancri, Eta Cancri and Gamma Cancri. Consequently, Delta Cancri itself is known as 鬼宿四 (Guǐ Su sì, English: the Fourth Star of Ghost.)
Delta Cancri was involved in the first recorded occultation by Jupiter:
"The most ancient observation of Jupiter which we are acquainted with is that reported by Ptolemy in book X, chap. iii (sic), of the Almagest, ...when the planet eclipsed the star known as (Delta) Cancri. This observation was made on September 3, B.C. 240, about 18h on the meridian of Alexandria."— Allen, 1898, quoting from Hind's The Solar System).
Delta Cancri also marks the famous open star cluster Praesepe (or the Beehive Cluster, also known as Messier 44). In ancient times M44 was used as a weather gauge as the following Greek rhyme from Aratos' Prognostica reveals:
A murky manger with both stars
Shining unaltered is a sign of rain.
While if the northern Ass is dimmed
By vaporous shroud, he of the south gleam radiant,
Expect a south wind: the vaporous shroud and radianceExchanging stars harbinger Boreas.— Allen, 1898
The meaning of this verse is that if Asellus Borealis or Gamma Cancris is hidden by clouds, the wind will be from the south and that situation will be reversed if Asellus Australis is obscured. There is some doubt however as to the accuracy of this as Allen notes: "Our modern Weather Bureau would probably tell us that if one of these stars were thus concealed, the other also would be." (Allen, 1898)
But Delta Cancri also acts as more than just a dubious weather guide: it is a reliable signpost for finding the vividly red star X Cancri as Patrick Moore notes in his guidebook Stars of the Southern Skies:
“In the same binocular field with Delta [Cancri] you will find one of the reddest stars in the sky: X Cancri. It is a semi-regular variable; at maximum it rises to magnitude 5 and it never falls below 7.3 so that it can always be seen with binoculars. It looks rather like a tiny glowing coal.”— Page 146, Moore, 1994.
Delta Cancri also marks the radiant of the Delta Cancrids meteor shower.
- Delta Cancrids (meteor shower)
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- "IAU Catalog of Star Names". Retrieved 21 November 2016.
- IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
- Allen, Richard Hinckley, Star Names: Their Lore and Meaning, 1898.
- (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
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- Kaler, 2009:"ASELLUS BOREALIS". Archived from the original on 2011-07-16.
- "The companion to delta Cancri". Astronomy Abstract Service. June 1876. Bibcode:1878Obs.....2...60B.
- Allen, Richard Hinckley, Star Names: Their Lore and Meaning, Dover, 1898.
- Burnham, Robert Jnr., Burnham's Celestial Handbook, Dover publications, 1978.
- Beresford Tony, Personal Communication (Letter on d Cancris.) 1997.
- Guinness, The Guinness Book of Records, Guinness Publishing, 1991.
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- Moore, Patrick, Stars of the Southern Skies, Penguin books, 1994.