Wandering through the realms of the cosmos, pondering its huge vastness

Transits and Occultations

When Luna Occults Subra

Last March 17, 2011 , the University of the Philippines Astronomical Society (UP AstroSoc) set up at the PAGASA Astronomical Observatory in UP Diliman to observe the  occultation of the 3.3 magnitude star, Omicron Leonis (or Subra) by the 92% illuminated waxing gibbous Moon.

This event was headed by UP AstroSoc associate member and alumni, Anthony Urbano of EtenyWorks. Kuya Eteny, as the members fondly call him, was experienced in observing occultations.

During this observation, he brought his 6″ Newtonian Equatorial Reflecting Telescope (NERT) with a self-designed home-built clock drive attached to the telescope’s equatorial mount. To record the occultation event, a Canon S3IS connected to a laptop was mounted to the telescope’s eyepiece by means of a fabricated camera adapter. This modified camera can show it’s system time on its on-screen display. According to Kuya Eteny, the default precision of the on-screen timer is limited to 1 second, but a patch, currently made available only for Canon S3IS, increased the clock’s precision to 1/100 of a second — the maximum precision of the camera’s built-in clock.

The telescopes set up which included the 6" NERT (left-most)

You can learn more about this improvised clock drive project, the camera modification and  the rest of observation set up by visiting his site where he posts a lot of cool stuff about observation and instrumentation. His inventions are most fit for those amateur astronomers interested in modifying their own telescopes and cameras especially for the purpose of doing astrophotography. 🙂

The event was from 10:20 UT (ingress) and ended at 11:10 UT (egress). Although it can be classified as a ‘bright star occultation’,  the light coming from the target star wasn’t bright enough to pass through the thick clouds during the entire event. By around 11:50 UT, we decided to packed up since there was still no trace of the star near the Moon.

On-screen display of Canon S3IS (with the CHDK firmware upgrade) showing the Moon under the dark patches of clouds. The system time (upper right corner) in this screenshot reads 19:18:44 PST (11:18:44 UT) The occultation event during this time was over, yet there was still no sign of Subra which should be located a few degrees above the Moons upper right limb.

When the Moon passes in front of a background star during occultations, the shadow of the Moon cast by the star sweeps across the Earth. When the leading or trailing edge of the Moon’s shadow crosses an observer, the observer sees the star “disappear” or “reappear”. These events are usually very sudden, and timing the instant of occultation is an important astronomical measurement.

But why is it important to observe lunar occultations?

  • Observing lunar occultations is important because the results improve our knowledge of the position and motion of the Moon. For example, when you time the disappearance of a star behind the edge of the Moon to 0.1 second accuracy (a value easily attainable), you are actually fixing the position of the Moon’s edge in space to an accuracy of about 80 metres. i.e. you are making a measurement with a precision of only 80 metres over a distance of 384,400 km. (This is one of the most accurate measurements an amateur observer can make in any branch of science!)
  • Combining many such measurements of the Moon’s position over a long time gives astronomers new information about the Moon’s motion and orbit. For example, total occultation observations have shown that the Moon is spiralling away from the Earth at a rate of a few centimetres per year.
  • Total lunar occultations have also been used to provide valuable information about star positions, about the hills and valleys on the edge of the Moon, and to discover new double stars.

Aside from occultations by the Moon, there were also Planetary Occultations and Asteroid Occultations. Just as the Moon passes in front of background stars, so too do planets and minor planets (also called asteroids).

Planetary occultations are occultations of stars by the passing of a planet in front of it. However planetary occultations occur less frequently than lunar occultations because the planets appear so much smaller in our sky than does the Moon. Nevertheless, observing occultations of stars by planets has yielded some stunning discoveries – for example, the rings of Uranus, and the atmosphere around Pluto.

On the other hand, Asteroid Occultations are occultations of stars by the passing of an asteroid in front of it. Asteroid occultations can occur anywhere on the surface of the earth. A few naked eye stars have been occulted during the past 20 years, but most occultations are of quite dim stars typically between magnitudes +9 and +12. An occultation might occur at any time of night, on any day of the week. More and more fainter asteroid occultations are being predicted, so that it is likely that at least 5 events will likely cross your area in the coming year.

While occultations of bright stars by major planets are very rare, occultations by asteroids are a little less so. This is not because any one asteroid has a greater chance of passing in front of a star. Rather, it is because there are so many more asteroids to choose from!

Anyway, asteroid occultations are the only way — apart from spacecraft missions to asteroids and radar observations of nearby objects — to determine the approximate size and shape of those bodies and are, of course, much cheaper.

If, as an amateur astronomer or telescope owner, you would like to be part of history, contribute something relevant to the study of astronomy, or would love to see sights that few have witnessed, then occultations are the thing for you. The occultation process offers discovery and research. It is possible for amateur astronomers to discover new companions of stars, help to improve the polar diameter of the sun and moon, identify the existence of possible satellites orbiting asteroids, to improve knowledge of heights of lunar mountain peaks and depths of valleys in the polar regions, determine corrections to ephemeris errors and assess star position errors, improve knowledge of the shape and sizes of asteroids, and more through occultation science. It does not matter where you live in the world. If you have access to a computer and possess a telescope of at least 4-6 inches, know your geodetic position either from GPS or a good topographic map, have a source of time signals and tape recorder, you can make your own observations of these rare and critical events.

The International Occultation Timing Association (IOTA) web site can be found here and that of the International Occultation Timing Association/European Section (IOTA/ES) web site can be found here.

 The IOTA web site contains predictions that are updated frequently.

To be able to observe and correctly record an occultation event, you should first have knowledge to find your way about the sky. Most stars that are occulted by asteroids have average apparent visual magnitude of 10.

The program Win-OCCULT, authored by David Herald in Australia, provides accurate predictions of all types of occultations and related phenomena. You can obtain a copy of Win-OCCULT by downloading it from here.

Good luck! 🙂

Lunar Occultation of Omicron Leonis — 17 March 2011

For Philippine observers, a star-moon eclipse will occur on March 17, 2011, from 6:20 pm to 7:10 pm (PHT). The event will be visible to the naked eye, but is best observed with a pair of binoculars, or with an astronomical telescope.

image reposted from eteny.wordpress.com

DETAILS (from AstronomyLive.com)

VISIBILITY: This will be the second time this year that OMICRON LEONIS or Subra will be occulted. This time it’s again visible in south-east Asia and the Phillipines but now also Australia and New Zealand can join the party! The star will disappear on the dark side, meaning easy to spot and very interesting to broadcast.

STAR & MOON: Omicron Leonis is a +3.5 magnitude star. The waxing Moon will be 92%, so there is some lunar disturbance. It all starts in Iba, the Phillipines at 10:50 Universal Time. Check the link below, to see when it starts and ends at your location.

LIVE BROADCASTING SCORE: Good. Although the Moon phase is not optimal, this occultation is beautiful to watch mainly because it disappears behind the dark side of the Moon. Another thing that makes broadcasting this occultation interesting is that the white line on the map – locations where the star will disappear and reappear behind the mountains on the lunar edge during darkness- is near the densely populated cities of south-east Australia. So when living near those white lines you should really find yourself a telescope and watch this occultation taking place.

HOW TO READ THE MAP BELOW?: The white line indicates that the occultation takes place during darkness. Blue is occultation at twilight and the dotted red line indicates occultation during daylight. Cyan means the occultation takes place when the Moon rises or sets. Another important feature of these maps is that you can check grazing locations (on the lines), places where the star will ‘scrape’ the Moon and thereby is occulted for a very short period of time.


Lunar Transit Captured by SDO – March 4, 2011


There is the Moon eating away at the Sun! 🙂 Image credit: NASA/GSFC/SDO


NASA’s Solar Dynamics Observatory (SDO) satellite captured the dark moon creating a partial eclipse of the Sun last March 2-4, 2011.

More still images here.

Watch HD Video.

These images, while unusual and cool to see, also have practical value to the SDO science team. Karel Schrijver of Lockheed-Martin’s Solar and Astrophysics Lab explains: “The very sharp edge of the lunar limb allows us to measure the in-orbit characteristics of the telescope e.g., light diffraction on optics and filter support grids. Once these are characterized, we can use that information to correct our data for instrumental effects and sharpen up the images to even more detail.”

On October 7, 2010, SDO observed its first lunar transit when the new Moon passed directly between the spacecraft (in its geosynchronous orbit) and the Sun.

ISS Solar Transit Observation – Manila, Philippines (Jan. 13, 2011)

Last 13 January 2011, an International Space Station (ISS)  Solar Transit event visible in some parts of Metro Manila occurred at around 8:30 am PHT (Philippine Time). The relative rarity of  ISS Solar and Lunar Transits here in the Philippines plus the fact that I were fortunate enough to be very near the path of visibility of this transit inspired me a lot not to miss this event.

ISS Solar Transit Visibility Path | obtained from http://www.calsky.com courtesy of Anthony Urbano

Along with my UP AstroSoc orgmates and some fellow amateur astronomers from RTU AstroSoc and from the  Philippine Astronomical Society (PAS), we set up our equipment to observe this passage of the ISS in front of the Sun’s disk at the Manila Observatory in Quezon City, Philippines.

Beemi, our "command center", under an umbrella

Setting up our equipment and preparing the Baader solar filters

Our two NERTs -- 6" Newton (black) and 4.5" Datascope (white)

I already have some experience in observing solar eclipses but this was my first time to try timing and capturing a solar transit. Unlike eclipses, ISS transits are more challenging to observe because they happen too fast. To give you an idea of how fast these events occur, here is a video clip which I got from Youtube showing the ISS transiting in front of the Sun’s disk. It was captured by students of the Westfalenkolleg Dortmund on August 7, 2010 at 17:31 pm.

The transit above lasted for 1.06 seconds. The one we were to observe will have a duration of about 0.91 seconds — too swift to be captured using cameras so we opted to record a video of it instead.

Following are the transit details including the timings. The data was obtained by our UP AstroSoc orgmate and team leader, Anthony Urbano from www.calsky.com.  All times are in PST (Philippine Standard Time) or UT+8.

Distance from center-line: 1.06km
Path Width: 13.6km maximum
Time of Ingress: 8h30m40.6s
Culmination: 8h30m41.20s
Time of Egress: 8h30m41.6s
Transit Duration: 0.91s
Separation Angle: 0.037°
Position Angle: 121.2°
Satellite at Azimuth: 123.6° ESE at culmination
Satellite at Altitude: 26.4° at culmination
Angular Diameter: 25.6″
Angular Velocity: 35.2’/s
Ground Velocity: 7,457m/s
Size: 73.0m X 44.5m X 27.5mSatellite Distance: 723.6km

In clock-face concept, the space station will appear to move toward: 8:58

And by the way,  Anthony Urbano or Kuya Eteny, as we fondly call him, is a gadget master. 😀 He has invented a lot of devices like a universal camera adapter that allows any point-and-shoot camera (and even SLRs) to be attached to any optical system (binoculars, telescope, microscope, etc.). He even brought one of his camera adapters during this observation to attach the video camera to his 6″ Newtonian Reflector Telescope. Moreover, he has also modified some gadgets  like DSLRs to be more useful in astronomical observations and astrophotography. This guy is truly remarkable for possessing such great talent! 🙂

video camera and the L-type universal adapter

Our amazing gadget master 🙂

An hour before the ingress, the sky around our location was partly covered in clouds. The sky condition got even worse as the predicted time approached. We waited for several more minutes, just in case the clouds clear up and the actual time of transit is later than predicted. In the end, we didn’t see any trace of the ISS due to thick cloud cover. We packed up and left at around 9:30 am.

Though we were not able to accomplish our goal of recording this transit event, the experience is always worth it.  😀 Thanks to Kuya Eteny for informing us about this event and inviting us to observe with him.

I’m looking forward to more transit and solar observations. 🙂


*Photos by Bea Banzuela


Update: Another group of fellow Filipino amateur astronomers from the Astronomical League of the Philippines (ALP) were lucky enough to observe and take images of the ISS transiting the Sun. Their observation report and images can be found here.