23 April

1996: On 23 April 1996, five months after its launch, the ESA/NASA SOHO spacecraft reached its final position in orbit around the Sun.

SOHO moves now around the Sun on the sunward side of the Earth, where it enjoys an uninterrupted view of the Sun, by slowly orbiting around Lagrange point L1.

The L1 point is perhaps the most immediately significant of the Lagrangian points, which were discovered by mathematician Joseph Louis Lagrange. It lies 1.5 million kilometres inside Earth’s orbit, partway between the Sun and the Earth. Lagrangian points are where all the gravitational forces acting between two objects cancel each other out and therefore can be used by spacecraft to ‘hover’.

SOHO remains at the L1 point, orbiting the Sun at the same rate as Earth. From this position, SOHO constantly monitors the Sun and, as well as conducting scientific studies, SOHO is also an early warning spacecraft. It can report violent solar storms that could damage communications and navigation satellites in orbit around our planet.

1827: On 23 April 1827, William Rowan Hamilton presented his 'Theory of Systems of Rays' at the Royal Irish Academy in Dublin. Although he was still an undergraduate, only 21 years old, his work is one of the most important works in optics, for it provided a single function that brought together mechanics, optics and mathematics. It confirmed the wave theory of light, which states that light is a form of energy that travels in waves.