Jupiter Occults Bright Star in Asia on 2008 February 13

 

 

On February 13th Jupiter will occult the brightest star since 1971 and the brightest one of the entire 21st century. This event is only observable from a limited zone in Asia. For Europe and Africa a favourable occultation will follow next year – see outlook at the end of this article.

 

 

 

 

The Star

 

Ain al Rami = ν1 Sagittarii = 32 Sgr = SAO 187426 = HIP 92761
Spectrum = K1 II

V magnitude = 4.86

B-V = +1.42
Binary component: 10.8 mag; extrapolated position: 3.6", 84° (aprox.)

 

 

 

The Constellation

 

The following chart shows the constellation a few minutes before the immersion of ν1 Sagittarii into Jupiter’s atmosphere. Io’s shaddow will enter the disk before the star occultation, Io itself will start the transit thereafter. At a distance of 14 arc minutes the 4.99 mag star ν2 Sagittarii (= 35 Sgr) may be too distant to be used as comparison star. By the way, Jupiter occulted that star in 1996, just one revolution ago!

 

 

 

 

The Occultation

 

The following diagram informs about all significant data, calculated with Dave Herald’s OCCULT software. 

 

 

 

 

The Zone of Observability

 

Due to Jupiter’s elongation from the sun of only 42 degrees, most of the Earth’s hemisphere facing the planet is sunlit. This condition only leaves a narrow observability zone between Jupiter’s rise and dawn. The area surrounding the north-eastern part of the Indian Ocean, the Bengal Gulf, is most favourable, but observation is also possible from central China and Mongolia.  

 

 

 

 

The Observation

 

Observation Period

Due to the limited observability zone the conditions are quite similar anywhere. The duration of the occultation will not scatter much around half an hour centered on 22h 57m UT on February 13th, early morning of Thursday, Feb14th, in Asia.

Start the observation several minutes before 22:40 UT. Expect brightness variations of the star while diving through layers of the Jovian atmosphere. Even significant flashes and blinks may occur over a period of a minute or so. If recording is stopped during the occultation, it should be restarted before 23:10 UT.

 

Contrast & Filters

The star is about half a magnitude brighter than an average square arc second of Jupiter’s disk. Choosing the appropriate telescope should give good seeing priority over large aperture.

Jupiter’s average B-V colour index is +0.83, the corresponding value of the star is +1.42. The differences in brightness and colour should slightly increase at the jovigraphic latitudes around 54°N, where the occultation occurs. An orange or red filter can further enhance the contrast, but certainly a methane band filter is ideal.

 

Comparison Stars

To detect even minor brightness variations during the atmospherical transit a comparison star is necessary. As mentioned above, ν2 Sagittarii would be ideal, but its distance of 14 arc minutes may be too large for many frame sizes. Because closer to Jupiter no suitable stars are available, Jovian satellites should be used instead. Io may be too close to Jupiter’s rim, when the star reappears. Therefore Europa, 2 arc minutes west of Jupiter, should be included.

 

Timing

Precise timing is highly desireable. Different techniques may be used, depending on local equipment or procedures. A GPS time inserter in combination with video equipment would be ideal. Acoustic recording of radio time signal is possible, too, for video as well as visual observations. Even optical recording of any time display before and after the observation without stopping the camera may work well. If you use a webcam, an acoustic time signal should be synchronized by recording it with a built-in microphone to be transferred to the computer via the same USB port.

But even if timing fails, never mind. Report your observation, it may be valuable nevertheless!

 

Resolution

From the photometrical data of ν1 Sagittarii an angular diameter of  just under one milli arc second can be deduced. This corresponds to 4 km at Jupiter’s distance and therefore confines the linear resolution at the planet.

Jupiter moves with rather high relative velocity: 29.8”/h, corresponding to almost 36 km/s. The narrow angle at which the star will dive through the atmosphere reduces the vertical component to about 21 km/s. Therefore a frame rate of 5 fps or slightly higher is just sufficient to preserve the resolution defined by the star diameter.

 

Remark:  Neither Jovian satellites nor the rings will be involved in the occultation.

 

For comparison - a reappearance at Saturn’s rim

Two years ago, on 2006 Jan 25th, Saturn occulted the 8 mag star BY Cancri. The team J. Lecacheux, E. Frappa, F.Colas, and P. Laques observed this event at Pic du Midi observatory. They recorded an impressive series of spikes during the star's emersion near Saturn’s south pole.

Jean Lecacheux explained: "We used the 1.05-m telescope at f/16, a I-filter (Schott RG780), and a tiny Watec 902H black-and-white camera plus a DV camcorder. A "1PPS" GPS allowed time-incrustation in the frames."

On Jan 27 Eric Frappa posted to Planoccult: "I've uploaded a 1.25 min video (7Mo - divx codec - www.divx.com) here:
http://www.euraster.net/results/2006/20060125-Saturn/20060125_Saturn_spikes.avi "

 

In case of any problem with that video, you may try this cropped and more compressed (185 KB) Microsoft MPEG-4 version:

 

 

 

 

Outlook

         

During the 21st century there are only two more occultations of stars brighter than 6 mag by Jupiter:

 

 

Again these occultations are separated by one revolution of Jupiter.

Both stars, 44 & 45 Capricorni, are quite similar and separated by just 25'. A remarkable coincidence!

 

 

 

Reporting

 

Please report your observation results to

I.L.O.C.                 ILOC@jodc.go.jp

Mitsuru Soma         S o m a M T [at] c c . n a o . a c . j p

IOTA/ES               H J B [at] o c c u l t a t i o n s . i n f o

 

The author kindly requests for reports about positive or negative experiences concerning any conditions, telescopes, filters, cameras, etc. Thanks in advance!

 

 

 

Clear skies and successful observations of this unique event!

 

Alfons Gabel

 IOTA/ES

 a l f o n s . g a b e l [ a t ] t - o n l i n e . d e