Three animations illustrating the scanning law envisioned for Gaia have been created. Each animation shows the celestial sphere as viewed from the "outside". Gaia is at the centre of the box, which just encloses the celestial sphere. (The image above shows extracts from the animations - the original animations (in mpg format) can be viewed by following the links below.)
The first animation (mpg (1.3M)) shows, in red, the path of the Sun in one year. The blue line is the vector from Gaia towards the Sun.
The second animation (mpg (2.3M)) shows the motion of the Sun as before, but also (in purple) the path of the spin axis of Gaia. The spin axis is always 50 degrees away from the Sun and moves in a circle about the Sun. If the Sun had remained fixed on the sky, the spin axis would have traced a cone around the solar direction. But because of the Sun's motion, the resulting path of the spin axis is a series of loops on the sky. Each loop takes about 70 days.
The third animation (mpg ( 5.4M)) shows, in addition to the Sun and the spin axis, the path of one of the Astro fields (in green). Only the first two months of the scanning is shown, and the spin of the satellite is shown with only one revolution per day (instead of four rev/day, as will be used for Gaia). Already after two months a large fraction of the sky is covered with scans that cross each other in different directions. After six months, the whole sky gets at least a three-fold coverage. Gaia is designed to scan the sky in this manner for at least five years, after which every point has been thoroughly criss-crossed, which allows to measure the positions, motions and parallaxes of the stars.
Animations courtesy L. Lindegren