Crepuscular Ray

What is crepuscular ray?

The light scattered by airborne dust, water droplets, air molecules or aerosols which provide a visible contrast between shadowed and illuminated parts of the sky is called a crepuscular ray.

This may happen either during sunrise or sunset, although not necessarily at these times.

The crepuscular rays appear to radiate outwards from the source of light due to the effects of distance and perspective, but they are actually parallel.

As observed from the earth or ground, the shadowed areas around the rays are formed by mountain tops or clouds which block the path of sun/moon light.

The artists have imagined this in the form of light being showered from the heaven by the God/s. But if we happen to see them from above the earth, say from the space, the viewing perspective gives a clear illustration of the crepuscular ray being parallel to each other.

One such phenomenon was observed over the Indian Subcontinent on 18th Oct 2011from the space station by the expedition29 crew of the International Space Station Program. A Nikon D2Xs digital camera using a 110mm lens and 4288x2848 pixel, CMOS sensor captured the phenomena, when the Sun was setting to the West (left side of the photograph) and a cumulonimbus cloud obstructed/shadowed the Sun rays which penetrated through a layer of haze beneath the CB cloud.

The following details may be noted:-

Mission:- ISS029

Date of Observation :- 18/10/2011

Time of Observation :- 113253 GMT

Nadir Point:- Latitude :- 24.0, Longitude 80.8 (Negative Numbers indicate south for latitude an west for longitude).

Nadir to Photo Centre Direction :- Sun Azimuth:- 256 (Clockwise angle in degrees from north to the sun, measured at nadir point) Spacecraft Altitude:- 374 Kms

Sun Elevation Angle:- 7 (angle in degrees between the horizon and the sun measured at the nadir point)

Orbit Number:- 2016.

(Nadir is a point on the celestial sphere directly below the observer diametrically opposite to the Zenith. Strictly it is nadir at a given point is the local vertical direction pointing towards the direction of the gravitational force at that location.

These types of photographs have a great value to the scientists and public and are available on the internet and can be viewed at the NASA/JSC Gateway to Astronaut Photography of the Earth.

The image provided here by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center has been cropped (by them) to enhance and improve the contrast.

Milind Phadke
+919890317557