The angular width of the Earth’s equatorial radius as seen from the centre of the Sun, when the Earth is at a distance of 1 au. Its value is 8″.794143. Historically, the solar parallax was the prime quantity giving the linear scale of the Solar System, and was derived from observations of bodies in the Solar System as seen from widely spaced observatories, for example of transits of Mercury or Venus across the Sun. The solar parallax is now derived from direct measurements of distances in the Solar System by radar and transmissions from spacecraft.