October 18-19, 2016 Grazing Occultation of Aldebaran

October 18-19, 2016   Grazing Occultation of Aldebaran

Link to Public Announcement page for this event

Handout Summarizing Event (Word file)            San Gabriel Reservoir Observing Site Powerpoint


On pages 50 and 51 of the August issue of Sky and Telescope, I described how lunar mountains would successively cover and uncover the disk of Aldebaran (a Tauri) for observers in a narrow strip around the northern limit of the July 29th occultation. Since the Moon was a crescent and the grazing occultation occurred on the dark side, it provided a rare opportunity to notice the effects of a distant star’s diameter without optical aid.  Here is the webpage summarizing the results from that event.   Aldebaran Graze – 29 July 2016

On Tuesday night, October 18th-19th, there will be another chance, but the 85% sunlit gibbous Moon will be much brighter, so that binoculars will probably be needed to see the star’s dance among the darkened lunar craters near the North Pole. This time, the graze zone is about 500 miles farther north, passing over the 2nd largest city in the USA. Besides Los Angeles, the path also passes over Denver, and not far from Las Vegas and Minneapolis; see the map on the p. 48 of the October issue of Sky and Telescope. The more northern location means that a larger part of the United States and Canada will see a total occultation, with predictions given in the link at the end of this article. Unlike the July event, there will be no interference this time from twilight or the daytime Sun. This event occurs in the middle of the night throughout North America.


The Graze

The most interesting views will be from a strip of land only a few hundred yards wide along the occultation’s northern limit. You can examine the precise graze path to street-level accuracy using the interactive Google Maps links we will soon be providing for this event at the end of the article.

Viewed from the narrow graze zone, the giant star should disappear and reappear multiple times as hills and valleys along the Moon’s northern limb cover and expose it. Most of these events will appear non-instantaneous, even taking up to a full second, due to Aldebaran’s angular size, subtending 40 meters at the Moon’s distance. Sometimes a sideways-speeding hill may cover only a part of the star’s face, causing an incomplete partial occultation.

400ft_Graze Profile 692SK5 2016 Oct 19

Profile of the Moon near Los Angeles CA

See the table below for the Google Map you should use, since the maps are different for different elevations above sea level

Denver1700m_Graze Profile 692SK5 2016 Oct 19

Profile of the Moon near Denver CO

See the table below for the Google Map you should use, since the maps are different for different elevations above sea level

As explained in the August issue for the July 29th graze, we don’t expect to learn much new from observing this month’s event, but it could be an interesting opportunity for public outreach, a rare astronomical phenomenon visible with common binoculars by thousands of people who live in the graze zone. In spite of the bright Moon, the October 18th-19th Aldebaran graze is one of the better events in the current series for populous parts of North America. By chance, hundreds of astronomers will be attending the American Astronomical Society’s Division of Planetary Sciences during the week of this event in Pasadena, California, only a few miles north of the graze zone; the International Occultation Timing Association (IOTA) will be publicizing the graze there.  Shown here is the graze path through the Los Angeles area.  The green line matches the horizontal solid lines in the images of the profiles above.  The 2 black lines outline the area where grazes will be visible with the area between the red lines showing the most events.



Since the graze zone shifts south by over 100 feet for each 100-foot increase in elevation above sea level, there are separate Google Maps for elevations at 100-foot intervals.  In some cases, the interval is 200 feet, so that the shift is 100 ft. or less, as long as the Google Map closest to the elevation in your area is used.  Thus, you need to know the elevation of your observing area; it can be obtained from Google Earth or some other mapping software with terrain data. For example, the streets of downtown L.A. have an elevation around 300 feet, so the 300-foot Google Earth should be selected. But if you observe from the 30th floor of a high building, that adds about 300 feet to your elevation, so then you should select the 600-foot Google Map. Selecting an observing site can be an iterative process, since if the Google Map of your first guess of the elevation shows that the site’s elevation is different by more than 100 feet, you should select another Google Map with an elevation that is within 100 feet.

The topography of the Los Angeles area may be important for this event. Santa Ana winds blowing from the desert may keep the sky clear even at the Pacific coast, but marine clouds frequently move in during the evening, blocking the view for lower elevations, sometimes reaching even the San Gabriel foothills. To escape marine clouds, observers might drive up Highway 39 (San Gabriel Canyon Rd.) to the Morris Reservoir, where the highway, conveniently on the west side (giving a good view of the Moon to the east) is around 1300 feet elevation. But the narrower best part of the graze zone, where twice as many occultation events are predicted, passes over the southern end of the San Gabriel Reservoir, with road elevations about 1600 feet. The road has several places to turn off, park, and set up a telescope. For those wanting the driest conditions, the path crosses Apple Valley at elevations over 3000 feet in the southern Mojave Desert. IOTA will probably have its main expedition a little farther east, to where the graze zone crosses Highway 247 (Barstow Rd.) north of Lucerne Valley; dirt side roads there promise sites with solitude and clear skies.

Hyades Stars

Four to two hours before Aldebaran reappears from behind the Moon’s dark limb, the 4th-magnitude Hyades stars 71 Tauri, ZC 677, and theta1 and theta2 Tauri will do the same for much of the US and Canada.

Detailed predictions for those will be found below.

For More Information

* IOTA will update this page on a regular basis with more maps and event details.

* Detailed predictions for most of the events mentioned here are listed for more than 1,000 places at lunar-occultations.com/iota/bstar/bstar.htm. The page for each star displays three long tables:  The bright-side disappearance (telescope with high power needed), the much easier dark-side reappearance (easy with binoculars for Aldebaran), and the locations of the cities. — David Dunham

David Dunham, a founder of IOTA , has led the coordination of occultation timings since 1962. Write him at dunham@starpower.net.

GFS Cloud Forecast     7Timer Cloud Forecast (Occult)

Site Elevation Google Maps

Pick the Google Map, from the list below, that most closely matches your site elevation above sea level.

Once you identify a place where you might observe, using either one of the static maps or a Google Map, find that place using Google Earth, setting the cursor on the location and, WITHOUT MOVING THE CURSOR, read the elevation given at the bottom following the latitude and longitude. If that elevation is within 100 feet of the elevation for the Google Map you used, you are done, but if not, find the Google Map for an elevation that is within 100 feet of that for the desired location, enter the appropriate offsets, and verify that the place is still between the dark gray lines on your new map. Hopefully, you won’t need another iteration to select a good site, or sites.  Download this zip file and extract Aldebaran Graze KML file for Google Earth.  You will need to have Google Earth installed on your computer to use this file.

Zones for specific areas are listed in the table below.

Use the offset values for the area where you plan to observe that are given in the last two columns of the table below

More specific information is given in the paragraph below.

 0 feet   100 ft    200 ft    300 ft    400 ft    500 ft    600 ft    700 ft    800 ft    900 ft    1000 ft    1100 ft    1300 ft    1400 ft

1600 ft    1800 ft   2000 ft   2200 ft   2400 ft   2600 ft   2700 ft   2900 ft    3000 ft   3100 ft   3200 ft

   3300 ft   3400 ft   3500 ft   3600 ft   3700 ft  3800 ft  3900 ft     4000 ft   4200 ft   4400 ft   4600 ft   4700 ft

  4800 ft   5000 ft   5100 ft    5300 ft   5400 ft   5500 ft   5600 ft   5700 ft   5800 ft    6000 ft   8300 ft

Grazing Occultation of ZC 692 – Aldebaran (2016 Oct. 18/19)

Video Record from 2 min. before to 2 min. after the central times given in columns 2 and 3 below.

Offset values that need to be entered in the boxes at the top of the Google maps are given in the last two columns. The values are in km, with positive values specifying distances southeast of the predicted northern limit, shown by the green line that should be ignored since no occultation will occur there. The zones will be between the dark gray lines that will be plotted when you use the values specified. The “Graze zone” is the zone where multiple events should occur; no occultation will generally occur only about 200m north of the northern edge of this zone. The graze zone is useful for determining if fixed sites (fixed observatories, homes, etc.) will have multiple events. The “Best zone” is where mobile observers should set up; it will have 1.5 to 2 times the number of multiple events that might occur in other parts of the “Graze zone”. In some cases, the “Graze Zone” is narrow so that it is the same as the “Best zone”; in that case, no values are given in the “Graze zone” column.  Most of the “Static Maps” are not available yet, but they will be added in a few days. The ones that are available (in the LA area, and main Denver region) are in blue type, you can just click on them to get the map. Most observers can just use the static maps without the need to use the Google Map files, but if you do use the Google Map files, we recommend that you try to reproduce one of the static maps, so you can verify your work.

Caution:  If actual observation site Elevation is more than 100 feet from the elevations shown, a revised Google Map should be used.

If you plan to observe from an area not mentioned in the table below, specify the area from which you want to observe in a message to dunham@starpower.net and copied to btimerson@rochester.rr.com and we’ll either send you the necessary files, or add the new area to our table below.

[Just a note: The profile, independent of elevation, is OK; it’s the limit line, and its plot on the Google Map, that changes]

Region of Interest


Oct. 19

Local Time Static Maps Moon Alt Moon Az Graze Profiles Google Map

Elev., feet

from above

Graze zone, km  


zone, km

Marina Del Rey – USC* 5:26.7 10:26.7 pm PDT CA_Marina_Del_Rey,


19° 82° CA_Marina_Del_Rey 100 0.6 / 3.5 1.05/


Downtown LA* 5:26.8 10:26.8 pm PDT CA_LosAngeles,


19° 82° CA_LosAngeles 300 0.65 / 3.5 1.05/


Alhambra – w. Monrovia* 5:27.0 10:27.0

pm PDT



19° 83° CA_Monrovia 500 0.7 / 3.5 1.05/



low foothills*

5:27.0 10:27.0

pm PDT



19° 83° CA_Monrovia 700 0.7 / 3.5 1.05/



high foothills*

5:27.0 10:27.0

pm PDT



19° 83° CA_Monrovia 900 0.7 / 3.5 1.05/


Morris & San Gabriel Res.* 5:27.2 10:27.2

pm PDT

CA_MorrisRes, CA_SanGabrielRes,


19° 83° CA_Monrovia 1400, 1600 0.7 / 3.5 1.05/


*Whole LA Basin region CA_LA_labelled
s.e. of Wrightwood 5:27.4 10:27.4

pm PDT


Best CA_LonePineValleyRd

20° 83° CA_CajonJct 4700 0.7 / 3.6 1.08/


n.w. of Cajon Junction 5:27.4 10:27.4

pm PDT


Best _CA_Hwy 138

20° 83° CA_CajonJct 4000 0.7 / 3.6 1.08/


west Hesperia

e. of I-15

5:27.5 10:27.5

pm PDT



20° 83° CA_CajonJct 3800 0.7 / 3.6 1.08/


central Hesperia 5:27.6 10:27.6

pm PDT



20° 83° CA_AppleValley 3200 0.7 / 3.6 1.07/


Apple Valley 5:27.7 10:27.7

pm PDT




Apple Valley Expedition   Map 1   Map 2

20° 83° CA_AppleValley 3000 0.7 / 3.6 1.07/


Calif. Hwy 247 5:27.8 10:27.8

pm PDT


Best_CA_Hwy 247

20° 84° CA_Hwy247 3000 0.7 / 3.6 1.07/


south of

Las Vegas

5:29.6 10:29.6

pm PDT



23° 85° NV_Searchlight 3400 0.9 / 2.8 1.07/


Page, AZ

US 89

5:33.0 11:33.0

pm MDT




26° 88° AZ_Page 4600




Conifer, CO (high) 5:40.2 11:40.2

pm MDT

CO_Conifer 32° 95° CO_Conifer 8300 1.10/


Valley Heirloom, CO 5:40.3 11:40.3

pm MDT

CO_Valley_Heirloom 32° 95° CO_Denver 6000 1.10/


n.w. Ken Caryl, CO (Hwy 67) 5:40.4 11:40.4

pm MDT

CO_nw_Ken_Caryl 32° 95° CO_Denver 5800 1.10/


w. Bow Mar, CO (Hwy 121) 5:40.4 11:40.4

pm MDT

CO_w_Bow_Mar 32° 95° CO_Denver 5600 1.10/


Sheridan – Aurora (Denver main) 5:40.5 11:40.5 MDT CO_Denver_area


32° 95° CO_Denver 5400 1.10/


s. Sioux Falls, SD 5:51.4 12:51.4

am CDT


Best SD_n_Beresford

40° 106° SD_S_SiouxFalls 1400 0.7/3.5 0.95/


n. Mankato, MN 5:55.7 12:55.7

am CDT



42° 110° MN_Mankato 900 0.7/3.5 1.00/


Elko New Market, MN 5:57.0 12:57.0

am CDT



43° 112° MN_Minneapolis 1100 0.7/3.5 1.00/


Farmington-Coates, MN 5:57.3 12:57.3

am CDT



43° 112° MN_Minneapolis 900 0.7/3.5 1.00/


Hastings, MN (Miss. River) 5:57.6 12:57.6

am CDT



43° 112° MN_Hastings 800 0.7/3.5 1.00/



Total Occultations of Other Hyades stars during this time period can be found on this page.

These events include:  ZC 661, ZC 669, ZC 671, ZC 677, and ZC 692 (Aldebaran).