C/1939 H1 (Jurlof–Achmarof–Hassel)

Lore & Background

The comet was discovered by many observers independently. The first report to reach astronomical authorities was that of O. Hassel, who found the comet on the evening of April 16 from Hokksund, Norway. On April 20, Sergey Belyavsky announced that he received reports from amateur astronomers Achmarof and Jurlof, who found the comet independently on the evening of April 15. At that time the comet was near its closest approach to Earth, at 0.68 AU, and five days past perihelion, with a solar elongation of 31°.

On April 18 the apparent magnitude was estimated between 3 and 4. Vinter Hansen described the comet as big and diffuse with a tail. The visual tail was about 1 degree long, while photographs showed the tail extending for 16 degrees. On 20 April the comet reached its northernmost declination at +44°, and U. S. Lyons reported it was visible with the naked eye. By April 21 the mean apparent magnitude was 4, with the tail photographically extending 8.5°, but the next day Waterfield noted the tail had nearly disappeared. On May 1 the magnitude was 5 visually and 5.8 photographically.

The comet reached its maximum solar elongation of 43° on 4 May, with magnitude between 5.6 and 6.5. On 7 May the photographic magnitude was 6, the coma about 3 arcminutes across, and the tail extended 2.7 degrees. By May 20 the comet had faded to magnitude 7.5–7.8. It was last detected on May 27 by H. Hirose at Tokyo Observatory, when it was at low latitude.

Reader's Guide

The comet's significance lies in its dual meteor shower associations, identified decades after its discovery. V. Guth noted in 1939 that the comet's orbit approached Earth's orbit at both ascending and descending nodes, on 4 August and 31 January, suggesting it could produce meteors. One shower, the theta Cetids (IAU #535), was first identified in 2014 and observed from 2010 to 2020 without outbursts, active between 3 and 8 August alongside the Perseids. About a dozen meteors were recorded globally in 2019 and 2020. The other shower, the 47 Ophiuchids, is active from January 31 to February 13, with three meteors detected by the Croatian Meteor Network and SonotaCo Meteor Network between 2007 and 2013. The comet's orbital period is uncertain, with estimates ranging from 490 years (based on 14 observations over 28 days) to 6,490 years (based on 399 positions). Its legacy includes providing a rare link between a historical comet and modern meteor shower observations.

Did You Know?

• The comet was discovered independently by multiple observers, with the first report from Olaf Hassel on April 16, 1939.
• Its visual tail was about 1 degree long, but photographs showed a tail extending 16 degrees.
• The comet has a minimum orbit intersection distance with Earth of 0.013 AU (1.9 million km).
• Two meteor showers are associated with it: the theta Cetids and the 47 Ophiuchids.