Editing Comet Ikeya–Seki (section)

== Discovery and observations ==
[[File:Mensen op de brug bij de Amstel wachten tevergeefs op het zichtbaar worden van d, Bestanddeelnr 918-3461.jpg|thumb|left|Onlookers in [[Amsterdam]] awaiting the comet's apparition on 21 October 1965|alt=Black-and-white photograph of a crowd observing the comet]]
Comet Ikeya–Seki was independently discovered on 18 September 1965 by Japanese amateur astronomers [[Kaoru Ikeya]] and [[Tsutomu Seki]] within roughly 15&nbsp;minutes of each other. The recent passage of a [[typhoon]] had afforded [[Astronomical seeing|favorably clear]] conditions for identifying the comet.{{r|Hale_2020}} Upon discovery, the comet was an 8th-[[apparent magnitude|magnitude]]{{efn|Unless otherwise noted, all observations of comet magnitude refer to [[apparent magnitude]] in [[visible light]]. A more negative value for apparent magnitude indicates a brighter object.}} object visually located 10° west of [[Alphard|α Hydrae]],{{efn|Reported initial position upon discovery was: [[right ascension|α]] {{=}} {{RA|8|45.4}}, [[declination|δ]] {{=}} {{DEC|–8|38}}{{r|IAUC_1921}}}} moving east across the sky at around 1° per day;{{r|Hale_2020|Roemer_1965}} [[Proper motion|acceleration]] of the comet and significant brightening was observed shortly afterwards. Based on preliminary estimations of Ikeya–Seki's orbit, [[Fred Whipple]] of the [[Smithsonian Astrophysical Observatory]] conjectured that Ikeya–Seki was a [[sungrazing comet]]. Subsequent and more accurate computations of the comet's [[ephemerides|orbital properties]] showed a close similarity between Ikeya–Seki and the [[Great Comet of 1882]], including the calculated [[perihelion]] of 0.0079&nbsp;[[astronomical unit|AU]] (1.2&nbsp;million km, 0.7&nbsp;million mi). Leading up to perihelion, Ikeya–Seki's [[light curve]] was also similar to the 1882 comet.{{r|Roemer_1965}} The comet brightened to magnitude&nbsp;+4 by 7 October, with a [[tail (comet)|tail]] extending to over 1° in length.{{r|Roemer_1965}} By mid-October, the tail had extended to a length of 10°.{{r|Hale_2020}} The comet's approach of the Sun visually placed the comet at increasingly lower [[horizontal coordinate system|altitude]] and brighter skies, leading to greater difficulty in estimating the comet's brightness as perihelion drew nearer.{{r|Roemer_1966a}} Nonetheless, Ikeya–Seki's increasing luminosity remained apparent;{{r|Hale_2020}} in the Southern Hemisphere, where visibility of the comet was most favorable around the time of perihelion, observers reported Ikeya–Sekit to be as bright as magnitude&nbsp;0 by 18 October.{{r|Roemer_1966a}} The comet brightened considerably in the 60&nbsp;hours after 18 October. By 20 October, the comet had become easily visible with the [[naked eye]] in daylight. Ikeya–Seki continued to brighten as perihelion approached, becoming comparable in brightness to the [[full Moon]]. The comet also projected a slightly curved tail; 2° of the tail was sufficiently visible to the naked eye with manual obscuration of the Sun.{{r|Roemer_1966a}}
In October–November 1965 the observation was conducted from [[Mauna Kea, Hawaii]]. One of the key findings from this study was the documentation of the comet's rapid brightening and the detection of fragmentation in its nucleus. The images obtained provided evidence of the comet's intense interaction with solar radiation and its effects on the comet's physical structure.{{r|Herring_1966}}

Ikeya–Seki reached perihelion at 21:18&nbsp;[[Coordinated Universal Time|UTC]] on 21&nbsp;October.{{r|Roemer_1966a}} As viewed from Earth, the comet and the Sun were separated by only a few [[arcminute]]s.{{r|Hale_2020}} Observations indicated that the comet's [[comet nucleus|nucleus]] began to break apart near the time of perihelion,{{r|Roemer_1966a}} with Japanese observers noting two small fragments detaching from the primary nucleus that later evaporated soon after.{{r|Hale_2020|Hirayama_1965}} The comet faded after perihelion as it receded from the Sun, with the [[coma (comet)|coma]] dimming to magnitude&nbsp;+3 by 26&nbsp;October. However, its tail elongated, reaching a length of at least 15° by 26&nbsp;October and reaching a maximum of nearly 30° in early November&nbsp;1965. While the fragmented nucleus of Ikeya–Seki had hitherto remained close together, by 6&nbsp;November two primary components had become visually distinct in both separation and brightness. Though the comet's coma had dimmed to magnitude&nbsp;7.4 by 27&nbsp;November, a tail spanning 10° remained visible to the naked eye.{{r|Roemer_1966a}} The comet dimmed below naked-eye visibility by early December.{{r|Hale_2020}} The two components of Ikeya–Seki's fractured nucleus remained apparent with increasing visual separation, moving apart at approximately {{cvt|14|m/s|mph}}; one was brighter but more diffuse in appearance than the other. Extrapolation of the observed positions of the two nuclei calculated by [[Zdenek Sekanina]] suggested that the nuclei broke apart on 26&nbsp;October. Similar calculations by H. Pohn of the [[United States Geological Survey]] yielded 26&nbsp;October as the date of separation, though Sekanina believed Pohn's calculation used different cometary fragments.{{r|Roemer_1966b}} By 1966, the two fragments were separated by nearly a full arcminute.{{r|Hale_2020}} Although Ikeya–Seki's brightness closely paralleled the Great Comet of 1882, Ikeya–Seki dimmed much more rapidly after perihelion;{{r|Roemer_1966c}} while the 1882 comet was observed for up to eight months following perihelion,{{r|Roemer_1966c}} the last photographs of Ikeya–Seki were taken prior to mid-February 1966,{{r|Hale_2020}} after which the comet became fainter than magnitude&nbsp;+13.{{r|Roemer_1966b}} Ikeya–Seki was indiscernible in a 60-second [[Exposure (photography)|exposure]] using a 40-inch [[reflector telescope]] at the [[United States Naval Observatory Flagstaff Station]] in mid-March 1966.{{r|Roemer_1966c}}