16 May 2000

Such congregations of planets are rare events, and they cannot be seen from the ground when close in the sky to the dazzling Sun. But the LASCO C3 instrument on SOHO uses a mask to blot out direct sunlight, and it has a wide enough field of view (15 degrees) to take in the four planets in the same picture.

The accompanying diagram shows orbital positions of the planets and SOHO in relation to the Sun, at the time of the picture. The diagram is not to scale. SOHO is stationed 1.5 million kilometres from the Earth, on the sunward side, and the Sun is about 150 million km away. The nearest planet in SOHO's image is Mercury, less than 60 million km beyond the Sun, while Saturn's distance beyond the Sun is more than 1400 million km.

All of the planets move across the LASCO instrument's field of view from day to day, as they follow their individual orbits anti-clockwise around the Sun. But while Mercury and Venus progress in the natural direction (right to left) Jupiter and Saturn appear to shift backwards (left to right) because of the faster orbital speed of the Earth and SOHO.

Built in Europe for a project of international cooperation between the European Space Agency and NASA, SOHO is equipped with instruments from European and American scientists. The LASCO coronagraph is made and operated by a transatlantic team led by the US Naval Research Laboratory.