Astronomer Michael Giacobini of Nice, France was the first to discover this comet near the constellation of Aquarius on 20th December, 1900. The comet was travelling eastwards and was noticed for the second time by an astronomer Ernst Zinner of Bamberg, Germany on 23rd October, 1913. He noticed it while observing variable stars near the star, Beta Scuti. Zinner made some important calculations about the features of this comet and was named Giacobini and Zinner jointly, after the two astronomers who discovered it.
Observations about this comet include:
- On discovery apparition on 20th December 1900, the comet had a tail, about 30 arcmin long.
- The coma was 3 arcmin across on discovery and was one arc minute in diameter. (1 arcmin=1/60 of a degree).
- No tail was reported after two months of its discovery.
- The nucleus was noted at about 2 km. during the latter days of its discovery, the comet started fading in brightness from the month of January onwards.
On its discovery, the magnitude of the comet was noted at 10. However, towards the end of December, it was revealed at about 10.5–11. The 1946 and 1959 returns were the brightest at magnitude 6 and 7 respectively. The return of the comet on 21st November, 1998 saw the comet reach maximum brightness at magnitude 8.5.
The perihelion distance (nearest to the sun) of comet 21P/Giacobini–Zinner is about 1.034 AU while its aphelion distance (far from the sun) is about 6.0 AU.
The orbital period of the comet 21P/ Giacobini–Zinner was noted at 6.46 years at the time of its discovery. However, it was later revised to 6.56 years, 6.42 years and finally, to 6.52 years.
Comet 21P/Giacobini–Zinner was recovered on 16th October, 1926 at Bergdorf.
Comet 21P/Giacobini–Zinner had 7 close approaches with planet Earth from 1900 onwards and had two close encounters with the giant planet Jupiter in 1958 and 1969. However, it is expected to make two close approaches to the Earth and one close approach to Jupiter in the first half of the 21st century.
The next perihelion date of comet 21P/Giacobini–Zinner is around 11th September, 2018. It had last appeared on 11th February, 2012.