Mike Kretlow
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The Triton Stellar Occultation of 5/6 October 2017

Pluto and Triton from spacecraft

Figure 1: Scaled images of Pluto (diameter ~2370 km) and Triton (diameter ~2700 km), taken by the New Horizons and Voyager 2 spacecrafts. Credits: NASA / JPL.



  Call 4 Observation: Join our Triton occultation campaign !

Updates on this page:

20170912@0200 : Update of Figure 7
20170915@1120 : Added contacts
20170918@2325 : Added link to MIT project page


Mike Kretlow and
Wolfgang Beisker

Bruno Sicardy



In the night 5/6 October 2017 Neptune‘s largest moon Triton will occult the 12.7 V-mag star UCAC4 410-143659, as seen from the US East coast, Northern Africa, and Europe. 20 years after the last successful monitoring of Triton’s atmosphere using stellar occultations and almost 10 years since the last documented occultation (observed on 21 May 2008), this event will be a new opportunity to gather data about Triton‘s current atmosphere state and possible changes. Moreover the increase and improvement in amateur observing techniques and the fact, that the shadow will move across an area with a rather high “telescope density”, gives hope for the best ever observed occultation of that interesting body.



Occultation predictions for this event are provided by several sources. For example by the ERC Lucky Star project group [5] and on this website, see also next Figure 3.


Figure 3: Prediction of the occultation by Triton on 2017 October 05 by the author using JPL‘s nep081xl+DE431mx ephemeris and Gaia‘s DR1 star position. The red dot marks the central occultation time (23:51:38 UT), the yellow dots corresponds to the time tics (seconds) on the southern shadow border.


Main occultation data and circumstances

Thursday October 5th, 2017 (night to Friday, 6th)
Observing time for Europe
23:48 UT +/- 10-15 minutes (suggested recording time)
Observing time for USA (East)
23:54 UT +/- 10-15 minutes (suggested recording time)
Star Position (Gaia DR2)
RA 22 54 18.4364 | Dec -08 00 08.318
Star Magnitude
12.7 V, 12.5 R, 12.4 I (from APASS), 11.4 J (from 2MASS)
Triton Magnitude
13.5 V, 12.9 R, 12.6 I, 12.2 J
Neptune Magnitude
7.8 V, 8.2 R, 8.9 I, 10.4 J
Magnitude Drop (central occ.)
~1.4 mag
Maximum duration
~161 sec
Shadow velocity
16.8 km/s
Geocentric distance
29.08 au
Moon: angular distance to star and sunlit
33.5° / 100%
Star identifier
GDR1 2610107907030969600
UCAC4 410-143659
2MASS 22541840-0800082
URAT1 410-421654


Observation campaigns

IOTA/ES will again cooperate with the Lucky Star project group in Paris (PI: Bruno Sicardy) in order to achieve a successful pro-am observation campaign here in Europe. One of the main goals will be to catch the central flash. The path were the central flash will be visible is about 100 km wide. At the time of writing the cross-track uncertainty is about the same (100 km). Similar efforts are planned by IOTA to coordinate the observations in the US.

This and further information (like prediction updates) will be available from a variety of websites, like the author’s webpage [6], IOTA and IOTA/ES websites [7,8] and other individual web pages [9]. Announcements (especially predictions updates) will be also posted on mailing lists (IOTAoccultations, planoccult, etc.).

Observers from Germany (and Europe in general) should contact either Mike Kretlow or Wolfgang Beisker from IOTA/ES if they plan to observe and / or for submitting their observations to us.

It is noteworthy that SOFIA (Stratospheric Observatory for Infrared Astronomy) is also scheduled to observe this occultation [10,11], supported and completed by an ground-based occultation observation campaign [12].


Some observational issues and hints

1.The angular separation to the full Moon will be only 33.5° at the time of occultation and the Moon will be higher in the sky than Neptune. It might help to erect some kind of “moon shade” to prevent moonlight entering the telescope. Mobile telescopes could also be placed in the (Moon’s) shadow of buildings, roofs etc.

2. The angular separation between Neptune and Triton will be ~11 arcs at occultation time. Operating at long focal length might be useful for a better separation of the planet and the satellite. If Neptune is the only reference object on the image (due to a small FOV), then be careful that the planet will not be saturated / overexposed (see next item).

3. Neptune will be much brighter than the star (5 mag!) and probably be saturated on many low-dynamic cameras. In that case it cannot be used as a photometric reference object. On the other hand there aren’t too many reference stars with comparable magnitude in the FOV (see Figures 4 & 5), especially if a large image scale is given. Therefore, cameras with a higher dynamic range (i.e. 12 Bit and 16 Bit) should be preferred if available.

4. Measure the target (occulted) star against the reference star(s) the night(s) before (or after) the event (when the star and Triton are clearly separated). This allows to calculate the contribution of the occulted star to the total flux (star + Triton) during the event, and finally, to subtract Triton's flux from the light curve.

It might help to make some experiments with your setup (camera, barlow lenses, filters etc.) before the occultation. Tim Haymes (UK) made some tests using Barlow lenses and filters, see his webpage


Figure 4: Sloan Digital Sky Survey (SDSS) red image of the target field, centred on the target star. FOV is 14.5' x 14.6'. Image created with the CDS Aladin sky atlas tool.




Figure 5: Sloan Digital Sky Survey (SDSS) red image of the target field. FOV is 2.6' x 2.6'.The image is centered on Neptune (N), the R=12.5 mag target star is the bright star marked with T. The red squares are Gaia DR1 star markers. In about 53'' distance (PA = 248°) to the target star is a R=15.6 mag star '1' (UCAC4 410-143656). Star '2' is even fainter (R = 18.5 mag). Image created with the CDS Aladin sky atlas tool.




Figure 6: View on Neptune, its rings and Triton for the time of (European) central occultation (23:48 UT). The box has a dimension of 1x1 arc minutes. The angular distance between the center of Neptune and Triton will be 11.3''. Figure created with the JPL PDS Neptune Viewer Tool.



Figure 7: Altitude and Azimut (N=0°, E=90°) diagram for the target star at (European) occultation time.

About Triton

Triton was discovered on October 10th, 1846, by the English merchant and amateur astronomer William Lassel (1799-1880), just 17 days after the discovery of Neptune by the German astronomer Johann Gottfried Galle (1812-1910). Triton is named after the Greek sea god Triton, the son of Poseidon.

With a diameter of 2700 km, Triton is the largest natural satellite of Neptune, and the seventh-largest moon in our Solar System. Its mean bulk density is 2.061 g/cm³. Triton orbits Neptune in a retrograde, almost perfect circular orbit with a sideral period of about 5.9 days in a mean distance of only 14.3 Neptune radii. In contrast to the Earth-Moon system the tidal interactions with this retrograde motion of Triton will cause it to spiral inward in a long-term scale. As soon as Triton will be inside Neptune‘s Roche limit this will result in a collision with Neptune and / or a tidal breakup of Triton, forming a ring system similar to that around Saturn.

Because of its retrograde orbit and its size and composition similar to Pluto, Triton is believed to be a dwarf planet captured by Neptune‘s gravity from the Kuiper Belt (Figure 1).

Similar to Pluto, Triton has a tenuous nitrogendominated atmosphere, driven by surface ices, primarily N2,CH4, CO and CO2. The atmosphere extends up to about 900 km above the surface and the surface pressure was in the range 14-19 µbar at the time of the Voyager 2 flyby. From stellar occultations [1] we know that Pluto‘s atmosphere changed significantly due to seasonal effects (doubling of atmospheric pressure between 1988 and 2002). Pluto's orbit is much more elliptical than that of the other planets, and its rotational axis is tipped by a large angle relative to its orbit. Both circumstances in combination causes this effects. 

As the heliocentric distance of the Neptune-Triton system doesn‘t change very much over a sideral period of about 165 yrs due to Neptune‘s nearly circular orbit, Triton seasons are caused by a combination of exceptional orbital plane and spin axis orientations and the influence of Triton’s orbit precession period (~680 yrs). Tritons orbit inclination wrt Neptune‘s equator is 157° (an inclination over 90° means retrograde motion) while Neptune’s axis is tilted by ~30° against its orbital plane. Thus Triton’s spin axis tilt wrt Neptune's orbit can vary between 127° and 180° (current value is 130°), giving it extreme seasons. The superimposition of Neptune's 165 yrs orbit revolution period and Triton ~680 yrs orbit precession period results in a double sinusoidal waveform as the Sun (or the latitude of the sub-solar point) shifts alternately north and south with a varying amplitude (see Figure 2). Currently the southern hemisphere of the satellite is being illuminated by the Sun, after centuries of winter time. Every 650 years a hemisphere is facing an “extreme solstice”, as it was the case in the year 2000 for the southern hemisphere, where the sub-solar latitude reached 50° south.

Figure 2: In the year 2000 Triton underwent an "extreme" solstice, where the sub-solar point reached 50 degrees South. This happens every about 650 yrs. Marked by a dot are following events (in time order): (1) The Voyager 2 flyby in August 1989, (2) the stellar occultations of 18 Jul 1997 and (3) 21 May 2008 and the upcoming stellar occultation of 5 Oct 2017 (4). Data computed with IMCCE‘s Miriade Service (http://vo.imcce.fr/webservices/miriade/).


From occultation observations in 1997 Elliot et al. [2] derived a global warming on Triton and a significant increase of atmospheric pressure since the 1989 Voyager 2 flyby.

It took further 10 years until another stellar occultation by Triton has been successfully recorded in May 2008 [3]. Unfortunately the geometry of the chords (two almost grazing chords at the southern limb) limited the derived astrometry and therefore, within a 3-sigma confidence level, no significant value of the atmospheric pressure (and possible changes since 1997) could be derived [4].



This work is based on an article by the author, published in the Journal of the British Astronomical Association, Vol. 127, No. 4, 194-195 (2017 August) [13]. The author is grateful for the permission to reuse the material.



[1] Sicardy, B. et al., Large changes in Pluto‘s atmosphere as revealed by recent stellar occultations, Nature 424, 168-170 (2003).

[2] Elliot, J.L. et al., Global warming on Triton. Nature 393, 765-767 (25 June 1998).

[3] Sicardy, B. et al., The Triton stellar occultation of 21 May 2008, EPSC 2008 abstracts (2008).

[4] Sicardy, Bruno, Personal communication (July 2017).

[5] http://lesia.obspm.fr/lucky-star/predictions

[6] http://astro.kretlow.de/?Solar-System---Occultations

[7] https://occultations.org/ and http://www.asteroidoccultation.com/observations/NA/

[8] http://www.iota-es.de

[9] http://www.stargazer.me.uk/call4obs/NextEvent.htm

[10] Person, M., A New Look at Triton's Atmosphere, SOFIA Proposal, Cycle 5, ID. 05_0125 (2016).

[11] https://eclipse2017.nasa.gov/content/sofia-triton-occultation-observations

[12] https://hubble.mit.edu/

[13] http://www.britastro.org/journal and http://www.britastro.org/digital



Last edited: September 20, 2017, 13:48

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