Editing Comet Ikeya–Seki (section)

{{Short description|Great Comet of 1965}}
{{About|the "Great Comet of 1965"|the 1967–1969 comet of the same name|C/1967 Y1 (Ikeya–Seki)}}
{{For|other comets discovered by Kaoru Ikeya and Tsutomu Seki|Comet Ikeya|Comet Seki}}
{{Infobox Comet
| name            = C/1965 S1 (Ikeya–Seki)<br>(Great Comet of 1965)
| image           = Comet C 1965 S1 Ikeya-Seki.jpg
| caption         = Comet Ikeya–Seki photographed by Maynard Pittendreigh sometime in 1965
| discovery_ref   = {{r|IAUC_1921}}
| discoverer      = [[Kaoru Ikeya]]<br>[[Tsutomu Seki]]
| discovery_site  = [[Japan]]
| discovery_date  = 18 September 1965
| designations    = {{nowrap|1965 VIII}}<br>1965f{{r|ICQ1}}
| orbit_ref       = {{r|jpl1|jpl2}}
| orbit           = [[Kreutz sungrazer]]
| epoch           = 7 October 1965 ([[Julian day|JD]] 2439040.5) 
| observation_arc = 115 days
| obs             = 101–119
| semimajor       = 91.6 AU (A)<br>103.7 AU (B)
| perihelion      = 0.007786 AU (A)<br>0.007778 AU (B)
| aphelion        = 183 AU (A)<br/>207 AU (B)
| eccentricity    = 0.999915 (A)<br>0.999925 (B)
| period=795 years (A){{r|barycenter-A}}<br/>946 years (B){{r|barycenter-B}}
| inclination     = 141.864° (A)<br>141.861° (B)
| asc_node        = 346.995° (A)<br>346.981° (B)
| arg_peri        = 69.049° (A)<br>69.034° (B)
| physical_ref    = {{r|ICQ2}}
| magnitude       = –10.0<br><small>(1965 apparition)</small>
| last_p          = 21 October 1965
| next_p          =
}}

'''Comet Ikeya–Seki''', [[Astronomical naming conventions#Comets|formally designated]] '''C/1965 S1''', '''1965 VIII''', and '''1965f''', was a [[List of non-periodic comets|long-period]] [[comet]] discovered independently by [[Kaoru Ikeya]] and [[Tsutomu Seki]]. First observed as a faint telescopic object on 18 September 1965, the first calculations of its orbit suggested that on October 21, it would pass just {{cvt|450,000|km}} above the [[Sun]]'s surface, and would probably become extremely bright.

Comets can defy such predictions, but Ikeya–Seki performed as expected. As it approached [[perihelion]] observers reported that it was clearly visible in the daytime sky next to the Sun. In [[Japan]], where it reached [[perihelion]] at local noon, it was seen shining at [[Apparent magnitude|magnitude]] −10.{{r|ICQ2}} It proved to be one of the brightest comets seen in the last thousand years, and is sometimes known as the ''[[Great Comet]] of 1965''.

The comet was seen to break into three pieces just before its perihelion passage. The three pieces continued in almost identical orbits, and the comet re-appeared in the morning sky in late October, showing a very bright tail. By early 1966, it had faded from view as it receded into the outer [[Solar System]].

Ikeya–Seki is a member of the [[Kreutz sungrazer]]s, which are suggested to be fragments of a large comet.