Editing Deimos (moon) (section)

== Orbital characteristics ==
[[File:Orbits of Phobos and Deimos.gif|frame|right|Orbits of [[Phobos (moon)|Phobos]] and Deimos (to scale)]]
Deimos's [[orbit]] is nearly circular and is close to Mars's [[equatorial plane]]. Deimos is possibly an asteroid that was perturbed by [[Jupiter]] into an [[orbit]] that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed.<ref name="burns" /> Both Deimos and Phobos have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly [[eccentric orbit]], and adjusting its inclination into the equatorial plane, most likely by a combination of [[atmospheric drag]] and [[tidal force]]s;<ref name="cazenave">{{cite journal |last1=Cazenave |first1=A. | author-link=Anny Cazenave |last2=Dobrovolskis |first2=A. |last3=Lago |first3=B. |year=1980 |title=Orbital history of the Martian satellites with inferences on their origin |journal=Icarus |volume=44 |issue=3 |pages=730–744 |doi=10.1016/0019-1035(80)90140-2 |bibcode=1980Icar...44..730C}}</ref> it is not clear that sufficient time was available for this to have occurred for Deimos.<ref name="burns" />
[[File:PIA17352-MarsMoons-PhobosPassesDeimos-RealTime.gif|thumb|left|250px|''[[Curiosity (rover)|Curiosity's]]'' view of the [[moons of Mars|Mars moons]]: [[Phobos (moon)|Phobos]] passing in front of Deimos in [[Real time (media)|real-time]] (video-gif, 1 August 2013)]]
As seen from Mars, Deimos would have an [[angular diameter]] of no more than 2.5 minutes (sixty minutes make one degree), one twelfth of the width of the [[Moon]] as seen from [[Earth]], and would therefore appear almost star-like to the naked eye.<ref name="richardson">{{citation|author-link=Robert S. Richardson|last=Richardson|first=R. S.|url=http://adsabs.harvard.edu//full/seri/ASPL./0004//0000214.000.html|title=If You Were on Mars|work=Astronomical Society of the Pacific Leaflets|volume=4, Leaflet No. 178|date=December 1943|issue=178 |pages=214–221|bibcode=1943ASPL....4..214R }}</ref> At its brightest ("full moon") it would be about as bright as [[Venus]] is from Earth; at the first- or third-quarter phase it would be about as bright as [[Vega]]. With a small [[telescope]], a Martian observer could see Deimos's phases, which take 1.2648<ref>{{Cite web|url=https://www.britannica.com/place/Deimos-moon-of-Mars|title=Deimos|author= ((The Editors of Encyclopaedia Britannica))|website=Encyclopedia Britannica|access-date=23 February 2019}}</ref> days (Deimos's [[synodic period]]) to run their course.<ref name="richardson" />

Unlike Phobos, which orbits so fast that it rises in the west and sets in the east, Deimos rises in the east and sets in the west, slower than Mars's rotation speed. The Sun-synodic orbital period of Deimos of about 30.4<!--- Sun-synodic revolution period, varies with the position of Mars on its eccentric orbit ---> hours exceeds the Martian solar day ("[[timekeeping on Mars|sol]]") of about 24.7<!--- Sun-synodic rotation period ---> hours by such a small amount that 2.48 days (2.41 sols)<!--- Deimos is visible for 163.35° of its orbit from an equatorial vantage point ---> elapse between its rising and setting for an equatorial observer. From Deimos-rise to Deimos-rise (or setting to setting), 5.466 days (5.320 sols) elapse.{{citation needed|date=August 2020}}

Because Deimos's orbit is relatively close to Mars and has only a very small inclination to Mars's equator, it cannot be seen from Martian latitudes greater than 82.7°.<ref>{{Cite web |last=Miller |first=James |date=2018-02-01 |title=The Moons of Mars: Phobos and Deimos |url=https://www.astronomytrek.com/the-moons-of-mars-phobos-and-deimos/ |access-date=2024-08-29 |language=en-US}}</ref>

Deimos's orbit is slowly getting larger, because it is far enough away from Mars and because of [[tidal acceleration]]. It is expected to eventually escape Mars's gravity.<ref>{{cite web|url=https://www.space.com/20345-deimos-moon.html|title=Deimos: Facts About the Smaller Martian Moon|website=[[Space.com]]|date=8 December 2017 }}</ref>

=== Solar transits ===
[[File:Transit of Deimos from Mars.jpg|thumb|Deimos [[transit (astronomy)|transits]] the [[Sun]]{{Snd}} as viewed by the Mars rover [[Opportunity rover|''Opportunity'']] (4 March 2004)]]

{{Main|Transit of Deimos from Mars}}
Deimos regularly [[transit (astronomy)|passes in front of the Sun]] as seen from Mars. It is too small to cause a [[total eclipse]], appearing only as a small black dot moving across the Sun. Its angular diameter is only about 2.5 times the angular diameter of Venus during a [[transit of Venus]] from Earth. On 4 March 2004 a transit of Deimos was photographed by Mars rover ''[[Opportunity (rover)|Opportunity]]'', and on 13 March 2004 a transit was photographed by Mars rover ''[[Spirit (rover)|Spirit]]''.{{citation needed|date=August 2020}}

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