kanabona.com on drupal 7

celestial sphere: ring of fire

Submitted by shehal on Tue, 01/19/2010 - 17:01

On 15 January 2010 Sri Lanka witnessed one of the most beautiful astronomical events - a solar eclipse. Something interesting about this eclipse was the ring-like appearance during totality (and hence the name Annular Solar Eclipse). We (me and my wife) drove to Jaffna to observe and photograph this annular solar eclipse. We teamed up with the Astronomical Association of Sri Lanka (AALK). They had organized a eclipse watch site at the Hindu College Grounds1. This blog post answers some of the eclipse related questions various people (including police officers) asked me during the eclipse watch and on our way back to Colombo.
Ring of Fire

Why is there a ring? Isn't the moon supposed to completely cover the Sun?

Let's look at the geometry of the Sun-Moon-Earth alignment. Earth orbits around the Sun in an elliptical path (oval-shaped and not in a circular path) whree Sun is not at the center but at a focus3. Therefore, the distance to the Sun varies with each day.

The ratio between the distance and the size of the objects gives the apparent size of the object. If you were to hold a coin 25 centimeters away from you and hold the coin 1 meter way from you, you will notice that the coin becomes smaller.

The average distance to the Sun is 149,600,000 kilometers4. The (average) diameter of the Sun is 695,500 kilometers4. The average apparent size of the Sun can be calculated by dividing the diameter of the Sun by the distance. That is, 695,500 / 149,600,00 = 1/215

A standard Sri Lankan one rupee coin is 20mm in diameter5. If you held a one rupee coin 4.3 meters away, the coin would appear as big as the Sun. This is because the size-to-distance ratio is the same (20/4300 = 1/215).

The average distance to the Moon is 384,467 kilometers4 and the diameter of the Moon 1,737 kilometers4. The apparent size of the Moon would be 384,467 / 1,737 which is 1/221.

If you held a one rupee coin 4.4 meters away, the coin would appear as big as the Moon.

But what if you held a coin 4.4 meters way in-line with the Sun. Would the coin cover the Sun? If you are to cover the Sun with a one rupee coin the you should hold it no greater than 4.3 meters away from. Otherwise the coin will appear smaller than the Sun and you will be able to see the Sun. Similarly, average apparent size of the Sun is 1/215 which is slightly bigger than the apparent size of the Moon (1/221). That is, if the average Moon eclipsed the average Sun then we would see a small ring around it.

The minimum Earth-Sun distance is 147,100,000 km6 and the maximum distance is 152,100,000 km6. The maximum apparent size would be 1/211 and the maximum would be 1/218.
The minimum Earth-Moon distance is 363,300 km7 and the maximum distance is 405,500 km7. The maximum apparent size would be 1/209 and the maximum would be 1/233.

Thus there could be Solar eclipses where the Sun appears bigger than the Moon, the Moon appears bigger than the Sun and where both the Sun and the Moon are of the same size.

When the Moon appears smaller than the Sun, the Sun will appear as a ring. The 15 January 2010 eclipse was such event.

On 15 January 2010, the Earth was slightly closer by about 2 million kilometers. That is, the Sun was about 147,152,000 kilometers2 away during the time of the eclipse and the Moon was about 400,000 kilometers2 away. This made the Sun appear at 695,500 / 147,152,000 = 1/211 and the Moon appear at 1/230. Thus, the Sun appeared about 9% bigger than the Moon and hence the annular Solar eclipse.

The eclipse was visible in Colombo. Why did you go all the way to Jaffna?

Total Solar Eclipse of March 29, 2006 viewed from IIS
Photo Credit: NASA / IIS Astranaut

In a Solar eclipse the Moon casts a shadow on Earth. This shadow has a region called the Umbra which is the dark shadow and a Penumbra which is a lighter shadow. A total eclipse is visible where the Umbral shadow is cast and a partial eclipse is visible on regions where the Penumbral shadow is cast. The following map shows a path of the Umbral shadow. The yellow line is the center-line of the shadow while the red line is the lower or southern limit.

Eclipse Path over Sri Lanka
Source: NASA Eclipse Website

The reason why we drove 450+ km from Colombo to Jaffna is because we wanted to photograph the eclipse on the center-line or close the center-line as possible. We could have photographed the eclipsed anywhere above the red line. But the eclipse-ring would not be circular.

Other Eclipse Links

Photos

Note: all "Red" photographs were taken using an Infrared (IR) filter. And the Sky DID NOT turn red. :) The IR filter passes through all IR and some Red light through while blocking UV and visible light.

More photos available in my Flickr. Click here to visit the eclipse album.

C1: First Contact

First Contact (C1) is when the Moon first 'contacts' or obstructs the Sun. This occurred around
05:48 UT / 11:18 Local Time.

Partial Eclipse


Lens Flare

The lens flare is intentional. We attached a glass filter and a polarizing filter for multiple reflections.

Jaffna Library

Jaffna Clock Tower

Nallur Kovil

C2: Second Contact

Total (annular) eclipse has begins. The Moon has completely obstructed the Sun. This occurred around
07:50 UT / 13:20 Local Time.

Ring of Fire - The Annular Eclipse

C3: Third Contact

Total (annular) eclipse has ended. The Moon has ended its complete obstruction but it is partially obstructing the Sun. This occurred around 08:00 UT / 13:30 Local Time.

C4: Fourth Contact

Eclipse ends. Next eclipse in 2019. We wait till then. Eclipse ended at 09:42 UT / 15:12 Local Time.

References

  1. Hindu College Grounds on GoogleMaps
  2. Calculated using a tool called HORIZONS developed by NASA. HORIZONS generates a "ephemerides" for objects in the solar system.
  3. Kepler's Laws of Planetary Motion
  4. World Book at NASA
  5. Sri Lankan Rupee
  6. Sun Fact Sheet
  7. Sun Fact Sheet
  8. The International Space Station (ISS) was in position to view the umbral (ground) shadow cast by the Moon as it moved between the Sun and the Earth during the solar eclipse on March 29, 2006. This astronaut image captures the umbral shadow across southern Turkey, northern Cyprus, and the Mediterranean Sea. People living in these regions observed a total solar eclipse, in which the Moon completely covers the Sun’s disk. The astronaut photograph was taken at approximately 2:00 p.m. local time. The terminator of the eclipse—the line between the light and dark parts of the Sun’s disk— is visible as it passes across central Turkey. This total solar eclipse was the fourth to have occurred since 1999. The portion of the ISS visible at image top is the Space Station Remote Manipulator System.
  9. Eclipse Path with Durations - Sri Lanka
blog comments powered by Disqus