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[Centaur]

The 93% illuminated waxing gibbous Moon will occult magnitude +2.9 Pi Sagittarii for North American and northern South American observers during the afternoon and evening of 2011 AUG 10. This will be an evening event only for those in the eastern half of North America, the Caribbean and far northern South America. For those in western North America, the event will occur in the afternoon. Weather permitting, both the immersion and emersion will be visible during the evening from Chicagoland.

Observers on a graze path will be able to witness the star winking on and off as lunar hills and valleys pass by. Those between the graze paths will see a full occultation. Immersion will generally occur on the lunar dark limb, while emersion should come from the bright limb. The star’s apparent equatorial coordinates at the time of the event will be RA 19:10:29.6 and Dec S 21°00’10”.

I’ve created a graze map for the Americas and a preview graphic with data for observers at Chicago’s Adler Planetarium. They can be seen by clicking www.curtrenz.com/moon

Photos and descriptions of the event would be welcome additions to this thread. If you would like occultation or graze data specifically for your location, then please reply to this post with a longitude, latitude, time zone and altitude above sea level.

[Chicago Astronomer - Astro Joe]

Hmmmm...

This will occur during a Chicago Astronomer star party.

I would like to video this, but will be difficult unless I have another scope or fellow Chicago Astronomers to attend to the visitors.

[Centaur]

Aug 9, 2011 10:11:17 GMT -6 Chicago Astronomer - Astro Joe said:

Hmmmm...

This will occur during a Chicago Astronomer star party.

I would like to video this, but will be difficult unless I have another scope or fellow Chicago Astronomers to attend to the visitors.

I hope you will be able, Joe. Timings at the Adler would be appreciated to verify the accuracy of my calculations.

[Paulie pchris00]

If I'm not clouded out, I will also be getting timings, but I'm not sure where I'll will be. I'll send you LAT/LONG/ALT data after the observation to see if my timing data concurs with your predictions.

[hillimolly]

do u want to get there early to set up

[Paulie pchris00]

Wednesday, August 10, was a work night, so I couldn’t make it to the Chicago Astronomer session. Besides, I wanted to observe the Pi Sagittarii occultation uninterrupted. Hillary wanted to see the occultation too, but the Moon was low, and she has tall trees behind her house. We went to Gilbert Park in Portage for an unobstructed, uninterrupted observation.

My plan was to document the immersion and emersion times through multiple telescopes and cameras. Without practice, though, nothing went according to plan. My 4.5” Newtonian is too badly out of collimation for useful observation at high magnification, but I wanted to use it. I figured I could run my Mead camera to my computer to help keep an eye on Pi Sagittarii while I was away from the eyepiece of my 6” Dob. That probably would have worked, but the batteries were almost dead, and the scope wasn’t tracking properly.

The plan for my Dob was to get pictures with one of my Logitech webcams, since the webcam images have a timestamp. I had forgotten that my testing of the webcam for lunar images hadn’t been good, and wasn’t getting quality images. As a last resort, I was forced to use my Vivitar point and shoot camera, without a timestamp for each image. Something is better than nothing.

As soon as I had the Dob ready to go around 8:30 PM CDST, I could see Pi Sagittarii already near the northeast lunar limb. Hillary could also see it with her 60mm shorty refractor. I didn’t have much time before Curt’s predicted immersion, and was spending more time trying to eliminate problems than observing. As Luna drew ever closer to Pi Sagittarii, I had to focus on getting whatever I could, and be ready to check the time as soon as the star disappeared behind the lunar limb. I had a small window open with a Cartes du Ciel time displayed in CDST, and UTC for my timings.








Webcam images. Not as good as I was hoping they would be.









I was expecting immersion ahead of Curt’s predicted time, and really wanted to get a video of when the star disappeared. When the predicted time rolled around, the star was still visible. I was getting my camera ready for video, but wasn’t shooting yet when I couldn’t see Pi Sagittarii on the video screen anymore. I double checked at the eyepiece, and it was gone. The time was 8:53:50 PM CDST. I took a few pictures of the Moon while the star was occulted, then shut down my computer to save battery, and enjoyed the last of my Dairy Queen chocolate shake.







Pi Sagittarii nearing the dark, leading lunar limb.


I had another set of batteries for my go-to Newt in my astronomy case, but they too were dead, and that scope was rendered useless the rest of the observation. I didn’t do much observing throughout the occultation, and just kind of hung out with Hillary until the emersion time approached. The emersion was behind schedule, and I was diligently watching through my 25mm eyepiece, waiting. I picked it out immediately, and noted the time (on the back of my hand) as 9:51:04 PM CDST. My first sets of images after emersion weren’t very successful, but as the Moon moved further from it, Pi Sagitarii became increasingly easier to capture out of the lunar glare. After only a few minutes of watching the two separate, it was time to pack up, take Hillary home, and go to work.





I thought I was well prepared for a great observation.







Hillary waiting for Pi Sagittarii to emerge from behind the Moon.






Luna during the occultation.




Pi Sagittarii just after emersion.











This was the third time I’ve caught at least part of a lunar occultation. I’ve also timed transits and shadow transits of the Galilean Moons in the Jovian system. I haven’t documented any of them as well as I would like to, but they are all learning experiences. I use these events to help train myself for the BIG ONE, next year’s transit of Venus across the face of the Sun. I’d like to gather scientifically useful data that can be used to measure solar parallax during that event, according to the method of French astronomer Delisle. I think I can do it, and every time I document a lunar occultation or Galilean transit, I hone my skills and methods for that last chance at the Venus transit.

[Centaur]

Thanks for your fine report and photos, Paulie. Please keep in mind that occultation shadows sweep across the Earth’s surface from west to east. Therefore occultation events occur in Illinois before they occur in Indiana.

[patrickm]

Aug 21, 2011 20:34:36 GMT -6 Centaur said:

Thanks for your fine report and photos, Paulie. Please keep in mind that occultation shadows sweep across the Earth’s surface from west to east. Therefore occultation events occur in Illinois before they occur in Indiana.

Woah! Gotta think about that one a bit. So much for sleep.

Even in Daylight Savings Time?

[Centaur]

Aug 21, 2011 22:55:11 GMT -6 patrickm said:

Woah! Gotta think about that one a bit. So much for sleep.

Even in Daylight Savings Time?

Patrick, the Moon revolves around the Earth in the same direction as the Earth rotates on its axis. The Moon's absolute orbital speed is greater than the Earth's surface's absolute rotational speed, although the Moon's angular orbital speed is less than that of the Earth's rotation due the Moon's greater distance from the Earth's center. Hence the Moon's shadow during a solar eclipse or occultation sweeps from west to east, even though in the sky the Moon appears to move from east to west.

Northwest Indiana, where Paulie lives, maintains the same time zone as Chicago all year long.

[tomlee]

Hard to imagine but it must be the way it is. be nice to have illustration to aid.

[Centaur]

Aug 22, 2011 10:54:34 GMT -6 tomlee said:

Hard to imagine but it must be the way it is. be nice to have illustration to aid.

Another way of describing the situation, Tom, is that the Moon appears to move east to west relative to the local celestial dome due to the Earth’s rotation, but it appears to move west to east relative to the stars due to its orbital movement around the Earth. In other words, the stars appear to be moving across the sky east to west faster than the Moon.

[tomlee]

OMG I thought those pictures were taken yesterday. Flipped photos posted many days later, the whole directional thing have to turn around now that I see your referenced directions are different. Thanks Curt.

[Centaur]

Aug 22, 2011 18:44:35 GMT -6 tomlee said:

OMG I thought those pictures were taken yesterday. Flipped photos posted many days later, the whole directional thing have to turn around now that I see your referenced directions are different. Thanks Curt.

There is an ambiguity when describing east-west directions on the Moon. Traditionally for observers north of the tropics looking upward at objects in the southern sky, east is toward the left and west is toward the right. However, while training astronauts to reach the Moon, in order to make lunar directions similar to when looking down at a map of the Earth, NASA decided to employ a convention in reverse of the original. So there is one convention for traditional astronomy and another for modern astronautics. To add to the confusion, astronomical photos are sometimes displayed in the upside-down manner seen in a reflecting telescope.

[Paulie pchris00]

Damn Apollo astronauts screwed everything up.

After Curt's explanations and thinking this through, it makes sense. I was assuming that since I was east of Adler, the timings would be slightly earlier, just like sunrise times are slightly earlier.

I didn't mention east-west direction in my original post, and because of the ambiguity, I usually don't. I know that officially, the Sea of Crises is in the east, and the Ocean of Storms is in the west, but that's not how I describe it. I call it as I see it, and use the historical references to justify myself. For me, a first quarter Moon it lit on the western hemisphere, and a third quarter Moon is lit on it's eastern hemisphere.

Then there is the telescopic view to contend with. A Newtonian reflector, like I used for this observation, flips the image upside-down. That doesn't bother me at all, because I'm accustomed to it. A refractor with a 90 degree diagonal flips the east-west image; to me it's backwards. It screws me up, and I get lost, even though I'm quite familiar with the lunar surface features.




Let's use this picture as an example. The lunar north pole is pointing to the lower right, while the south pole is pointing to the upper left. Pi Sagittarii is very far to the north just before immersion, on what I call the eastern hemisphere. When it emerged about an hour later, it was farther south, off what I would describe as the northwestern limb, as seen below.





I don't usually include directional references in my lunar images. I guess I assume anybody seeing them here has some familiarity with Luna, and would recognize that they are upside-down. As confusing as the east-west directions can be in reference to our Moon, every other body in our solar system is worse. The Moon shows us a constant face. Let's consider two major bodies that do not show us a constant face, one that rotates slowly, one that rotates quickly.

Our Sun rotates (at the equator) once every 25(?) days. Using just one observation of the Sun, it would be difficult to deduce even north-south. Observing a group of sunspots over several days though shows that they are moving east to west (I think; somebody please correct if I'm wrong). We can observe rotational movement on Jupiter much easier, in a matter of hours. And even though the Southern Equatorial Belt occasionally disappears, as it did last year, the two main atmospheric belts are constant enough to tell north-south right away. Observation of any features in the Jovian atmosphere, like the Great Red Spot, will show the direction of rotation. The Galilean Moons can also be used, since they orbit in the same direction as Jupiter rotates.

I get confused by orbital motion and rotational motion all the time. Everything we observe is moving relative to Earth, and in our solar system, this is very easy to observe. Careful, repeated observations and thinking through the results of those observations helps it all make sense.