Goodbye Hesperus

Goodbye Eσπερος - Hesperus, the evening star, gives a final 2009 appearance. Images of near horizon Venus by:

1,2 Mohamad Soltanolkotabi
   Esfahan, Iran March 22nd

3 Doug Zubenel, Kansas, USA
   March 21

4 Audrius Dubietis, Vidiskes
   Lithuania March 12

We have seen bright Venus with pillars, oval or circular aureoles and coronae. Here, close to the western horizon, its thin crescent splits into prismatic colours.

Our atmosphere decreases in density away from the surface. Light rays from Venus are refracted as they pass down through the layers of increasing density and refractive index. The resulting ray curvature is towards the greater density and it makes objects appear slightly higher than they really are.

But blue and green light is refracted more strongly than red (dispersion) and the ‘green Venus appears higher than the red one. The result is that a thin crescent appears prismatic with the red side towards the horizon.

Stars show the same effect and it gives rise to the green rim of the setting sun.

Mohamad's image 1 shows an additional effect, scintillation. Moving pockets of air of different density cause rapid image movements and colour changes.