Monday, April 03, 2006

Pleiades Occultation: Celaeno Graze 1-Apr

Updated: Mon. am 17-Apr: Added link to an article by Mitsuru Soma ‘Limb Profiles of the Moon Obtained from Grazing Occultation Observations’ in the ‘Software and the Celaeno Graze’ section.

During the recent passage of the Moon through the Pleiades a remarkable event was visible within the borders of N.Y.C. — something I hadn't seen before.


Field of view of Cartes du Ciel image is ~2.5 degrees
Time = 6:00pm EST / Limiting magnitude = 8.0
(Click image to visit IOTA website)

A group of us, Charlie, Kin, Peter & myself, headed down to a spot near the Staten Island Ferry terminal, on the Staten Island side, to attempt a grazing occultation of the star Celaeno (16 Tauri) in the Pleiades.

At magnitude 5.45 this Pleiad is not usually bright enough to see naked eye within the city. With optics this star is rather bright.

Charlie Ridgway found a place for us to observe along a favorable portion of the graze path on Staten Island. All it took to get there was a ride on the subway, a ferry trip and a short walk to the ‘Staten Island September 11 Memorial’.

Unfortunately partly cloudy skies ruled the night. We were able to observe a few of the total occultations between cloud breaks but the main event was the graze of Celaeno around 7:15pm EST.

The wind was rather strong with gusts up to 33 mph recorded in Central Park. Sustained winds were close to 15 mph. [weather history]

Software and the Celaeno Graze

There is an excellent piece of software for predicting lunar occultations including the viewing of these spectacular grazes. The name of the program is ‘Occult for Windows’.

The image below (click image for a clearer view) shows the contour of the Moon's limb at the time of the Celaeno graze from Staten Island. The time frame shown is roughly 6 minutes along the horizontal line starting on the left at -3m (ingress) and continuing to the right at +3m (egress).

The spot chosen was probably 0.75 miles off the center-line — described by the blue line on the chart. Looking along this line there's a single gap between the time of ingress and egress. This would represent a single reappearance of Celaeno. But ... we clearly saw three reappearances based on the recording!

The lunar limb profiles will need modifying based on three observing stations — Staten Island, New York (U.S.A.); Edmonton, Alberta (Canada); Kagoshima, Kyushu (Japan). See Charlie's write-up for the timing of the graze showing the three reappearance/disappearances between initial ingress and final egress for 4-Apr & 5-Apr.


For this graze, at our location, the distance between an occultation of Celaeno by a single mountain peak (at the northern limit) and a total occultation (at the southern limit) is nearly 1.3 miles or ~6,800 feet. An even narrower path for the multiple reappearances we witnessed (excluding final egress) is probably less than 0.2 of a mile or 2,100 feet. This is based on the Watts charts which are continually being improved upon by observations by amateurs. Here is an article by Fred Espenak on the ‘Lunar Limb Profile’ that should prove enlightening. Also, check out the following article by Mitsuru Soma ‘Limb Profiles of the Moon Obtained from Grazing Occultation Observations’.


It was great that all four of us were able to witness this graze of Celaeno. The anticipation of not knowing when it would start or how many events would occur increased the excitement.

As the event was about to happen the area around the Moon was remarkably clear of clouds. We don't usually get this lucky in the city. Everyone was able to clearly see Celaeno's close approach to the Moon before the event started. Since the whole of the graze would happen on the dark portion of the Moon's limb it was an easy event to witness.

Three of us had binoculars: Charlie — Canon 15x50 IS's, Peter — Takahashi 22x60's and Kin — Pentax 20x60's. I had a Televue-76 (76mm refractor) viewing the event at 48x.

Charlie was making an audio recording along with a WWV short-wave radio timing signal. As the star disappeared & reappeared we would call out the event — Out! In! Back! Gone! Still gone! It was an amazing thing to see. Being at the exact right spot for an event with such a narrow path so close to home was incredible.

If there is ever an opportunity for you to view a grazing occultation of a bright star I highly recommend the experience.

Thanks to all who made it possible to view this graze.

Equipment Purchases

I made a few recent purchases to test my skills at taking photos through the Televue-76. Recording a video of the event was a missed opportunity. It will take a lot of practice getting it right. I'm looking forward to nights where the Moon is up with a few stars nearby to test exposures for still photos and short movies.

Here are some recent purchases:
William Optics DCL-52 Digital Camera Adapter Lens
• 2 Televue extension tubes - 2" and 3">
• step down adapter ring 55mm-52mm

Here are some pending purchases:
• shutter release for the FZ-30
• extra battery
• extra 2 gigabyte memory card

Additional reports
• Reports from team TotL members — Charlie's ‘Astronomical Observations’, Peter's ‘Top of the Lawn’
• Other reports — Above the Clouds, Catching the Sky, Space Weather Gallery


Tag said...

AHHHH! Thanks for the superb writeup. It's now much clearer than mud :^D It actually shows the profile of the mountain (or crater walls?).

We definitely lucked out on our location to squeeze within a 2/10 corridor.

Thanks to you & Charlie for working out some of the science.


Ben C. said...


As far as I can tell this is a profile of a single mountain on the limb of the Moon.

The legend states that 20 vertcal millimeters is equal to 1 arcsecond. At this scale a full image of 'Moon' would be more than 125 feet across!

What we saw was probably the passage of Celaeno behind irregularities at the top of a mountain.

Pretty amazing that the software and the work behind the software was able to place us in the right location.


P.S. - 2/10 corridor?

P.P.S. - should I take 'superb' with a grain of salt? :)

Tag said...

>P.S. - 2/10 corridor?
I was referencing...

>> An even narrower path for the double reappearance we witnessed (excluding final egress) is just 0.2 of a mile or 2,100 feet!

I didn't put the unit.

>> P.P.S. - should I take 'superb' with a grain of salt?

No, it's a very well written {blog-}article of that evening.