The meridian motion of the solar spot

The animation shows the annual movement of the bright image of the Sun during its transit on the meridian line. The deformation of this image is thus clearly highlighted from one solstice to another.

Curve showing , for a given position on the line, the amount by which the image will move within 24 hours. This displacement is at its maximum (about 30 cm in one day) around 25 January. It is at its minimum during the solstices.

The brightness of the spot remains constant but its image as received on the ground varies as its dimensions vary. That is why, in the summer solstice, when the spot is at its most compact, it also seems to be brighter than in the winter solstice, at the other extremity of the meridian line, where its faint brightness makes it hard even to see. Nevertheless, the actual total amount of light received is identical but the surface of the image is 27 times larger in the winter solstice than in the summer solstice.

Moreover, it may seem that the animation speeds up and then slows down; this is not in fact the case. This visual impression is due to two factors: the first arises from the horizontal projection which has the effect of giving the degree of height a much larger scale as we approach the winter solstice (1.87 m / degree) than at the summer solstice (21.24 cm / degree), so a change of one degree in altitude results in a movement of the image of the Sun which is almost nine times greater at the winter solstice than at the summer solstice. Furthermore, the movement of the Sun, its rising or falling in the sky, is not constant from one day to the next. It is virtually zero at the solstices and maximum at the equinoxes. This has the effect toward the summer solstice of virtually immobilizing the solar spot, because the two factors will combine to produce the greatest amount of reduction in the displacement of the sunspot as it crosses the meridian.

Click and drag to the right or left in order to move along the meridian line.