Spectacular Graze of Spica, 2024 November 27 - Updated 2024 December 31
by David W. and Joan B. Dunham, Eberhard Riedel, and Derek Breit
A description of our observations recorded near Paint Rock, Texas is given, with event times and locations
This great graze on the dark side of a 13% sunlit Moon was the best in North America in 2024, so the Dunhams planned an expedition to the graze path, shown on pages 173 and 174 of the RASC Observer's Handbook for 2024, and also on pages 1 and 2 of this IOTA grazes paper. This plain text file gave the Universal Time of central graze, and the circumstances (esp. the altitudes of Spica and the Sun) at 0.25 degree intervals of longitude along the path from N. Mex. to the TX coast. The profile lined up best near long. 107 W in New Mexico, but very clear skies were needed to see it since the altitude above the horizon there was only 3 deg. Unfortunately, it was very cloudy over that part of New Mex. so the graze was observed elsewhere; the IR weather satellite image for the time of the graze is here. The profile still gave at least 4 occultations where conditions (esp. star altitude) were better over the northern Houston area. Links to Google maps of the graze path for different elevations above sea level are posted after the Google Earth section. For parallel offset lines can be specified with the Google maps to specify the better multiple events zones that you can then zoom in on to select observing sites. In the IR weather satellite view above, clear skies are orange, while high and mid-level clouds are yellow, green, and blue. But fog and thick low clouds, like those that covered much of east Texas, are also orange, but usually a darker shade than the clear areas, so it's hard to tell those areas since they are similar to the clear areas. A better representation of the widespread low clouds is give by this airport surface reports map, where the filled blue and red circles show airports reporting skies overcast with low clouds. In spite of that, there were a few places with breaks in the clouds, as Egan was able to find for his successful observation. Extensive pre-event information is on this IOTA page that we set up for the graze, last updated three days before it occurred. Joan and David recorded 4 - 5 occultations of the star from 2 sites about 25 miles east of San Angelo, Texas and just s. of Paint Rock, and near Hwy US83. PyMovie light curves for the two stations are shown in this .pdf document by Joan that also gives the times of the contacts at both stations determined with PyOTE. The Dunham1 site (Joan with 8in SCT) was at long. -99.91815 deg., lat. 31.49531 deg. as determined with a GPS flash timer that gives coordinates to the nearest 0.00001 deg., a precision of about 1.3m, which should be good enough. The elevation above sea level was 1642 ft. = 500.5m, as determined for this location from Google Earth, in perfect agreement with the GPS-measured elevation of 500m. The Dunham2 site (David with a midi system with NE2 Runcam) was at long. -99 deg. 55.075' = -99.91792, lat. 31 deg. 29.742' = 31.49570 deg. as determined with an IOTA-VTI. The elevation above sea level was 1640 ft. = 499.9m, as determined for this location from Google Earth, in good enough agreement with the VTI-measured elevation of 497m. Dunham2 was 49m north of Dunham1. The only other observation of the graze that we are aware of was by Mark Egan who observed near Florence, Texas, near the coast about as far east as it was possible to observe the event from land. He observed visually and made no timings; he was amazed to see 4 occultations of the star (8 contacts). His account of the nail-biter observation is in this message (.pdf). __________ GOOGLE EARTH FILES GENERATED WITH GRAZPREP IOTA/European Section's Eberhard Riedel has generated several Google Earth (GE) files that allowed observers to select observation sites in narrow bands where the predicted lunar profile shows the maximum number of occultations of the star during the graze can be observed. Thanks to the detailed lunar topographic information derived from NASA's Lunar Reconnaisance Orbiter (LRO), these predictions are quite accurate, much more so than before the early 2000's when we had to rely on Watts charts. Riedel has used his GRAZPREP software to project the lunar profile on the surface of the Earth so that, for any location in the graze zone, you can predict how many occultations of the star will occur. Riedel wants others to use GRAZPREP; his manual gives more information about how it can be done and its advantages as a modern graze prediction software package. Besides the GE files showing the whole profile, there are other GE files that show just the narrow bands where the msximum number of occultations (or contacts, with D and R, twice the number of occultations) will occur; they are shown as blue lines, with thicker yellow bars showing just the part of the lunar profile that will cause the occultations. A white line shows the "smooth (spherical) Moon" southern limit line for the whole occultation, with thicker white bars showing the part of the lunar profile that will occult the star there, usually fewer times than along the blue/yellow lines. I only include files for the "best" location near long. 107 deg. W. in New Mexico; for Houston, TX (for Egan's observation); and closest to our site near long. 100 deg. W. (Paint Rock, TX, east of San Angelo). This .zip file contains GE files for the graze path over three regions but only two regions are shown since it was cloudy and no observations were obtained near Austin [clicking on them shows an overview map showing the best band(s) with the most predicted contacts]: I-25 and NM Hwy 1 27 miles north of Truth or Consequences, NM (Long. 107 W) northern side of Houston, TX (Long. 95 W) This shows the whole profile ("all lines each 50m") for part of Houston. This .zip file contains GE files for the graze path over the San Angelo, TX region [the overview map showing the best band(s) with the most predicted contacts is shown in the maps of the Dunham sites east of San Angelo, see above in the description of their observations]. The GE files that you get from unzipping the .zip file (save them to a directory that you might name "Spica graze" on your computer), have long names including the central longitude for which they are valid and the elevation above sea level (in meters) and whether they show "all lines" or just the best band(s) as "X contacts" where "X" is the maximum # of contacts. You should just use the "X contacts" file for finding observing sites in your selected area. The "ZC 1925" in the name refers to the Zodiacal Catalog number of Spica. When you use Google Earth, when you zoom in on an observing site, you can toggle the line with the yellow bars off (or on) by clicking on the box shown in this screen view; that will allow you to see your site that might otherwise be covered by the yellow bar. __________ GOOGLE MAP PAGES GENERATED WITH OCCULT4 These were posted by Derek Breit, but we found that if we provide hot links to them, the old Java script doesn't work and you can't plot parallel offset lines, the main advantage of using the Google maps. Therefore, we provide the link URL's, but not as hot links. You need highlight and copy the link, then paste it into your Web browser; the Google map should then work, with the dark grey parallel offset lines that you specify in the two boxes above the map. To get values (in km) for the offsets, consult the predicted profile (links to them are given) to get the distances of the edges of the band you want to show. Note that the values you specify are negative north of the predicted limit line, and positive for south of it, as labelled on the vertical axis of the profile. Number of events are indicated as grey bars on the left side of the profile; the farther to the right it extends at a given distance, the more contacts will occur; just measure the top and bottom of the bar that extends farthest to the right. That can be done more accurately if you have Occult4 on your computer and generate the profile yourself, since then clicking anywhere on it gives the values. Below I list the longitude (clicking on it shows the predicted profile for that area), the U.T. and circumstances of central graze, the elevation above sealevel of the area, Derek's Google map link, and a description of the region (for more description, see the above Google Earth section for the corresponding longitude/area). Altitude (alt.) above the horizon is in degrees. elev. west U.T. Spica above Long. h m alt. sealevel Google map URL (link) Valid area 107.2 10:53.8 3 1600m http://www.poyntsource.com/New/Google/ZC1925_2024_Nov_27_1600m.htm NM Hwy1 - I-25 n. of Truth or Conseq., NM 100.0 10:56.3 10 500m http://www.poyntsource.com/New/Google/ZC1925_2024_Nov_27_500m.htm Paint Rock, TX 95.5 10:58.8 15 25m http://www.poyntsource.com/New/Google/ZC1925_2024_Nov_27_25m.htm n. side of Houston, TX ________________________________________________________ David Dunham, dunham@starpower.net Joan Dunham, dunhamjoan@verizon.net Eberhard Riedel, E_Riedel@msn.com Derek Breit, breit_ideas@poyntsource.com