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. The comet then was near its closest approach to Earth, at a distance of 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. Visually the tail was about 1 degree long, while in photographs it extended for 16 degrees. On 20 April the comet reached its northernmost declination at +44°. U. S. Lyons reported naked-eye visibility. On April 21 the mean apparent magnitude was 4, with the tail extending photographically for 8.5°, but the next day Waterfield mentioned the tail had nearly disappeared. By 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, coma about 3 arcminutes across, tail extending 2.7 degrees. By May 20 it had faded to magnitude 7.5–7.8. The comet 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 association with meteor showers, as noted by V. Guth in 1939. The comet has a minimum orbit intersection distance with Earth of 0.013 AU, and Guth observed that its 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, with meteors observed in all years from 2010 to 2020 without outbursts; about a dozen meteors were seen globally in 2019 and 2020. This shower is active between 3 and 8 August, alongside the Perseids. A second 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 link between a historical comet and modern meteor observations.

Did You Know?

• The comet was discovered independently by multiple observers, with the first reports from Olaf Hassel, Achmarof, and Jurlof.
• On April 18, 1939, the comet's visual tail was about 1 degree long, but photographs showed a tail extending 16 degrees.
• Two meteor showers are associated with this comet: the theta Cetids and the 47 Ophiuchids.