Saturday June 9th two of the outer planets, Mars and Saturn, will be at the same heliocentric longitude of about 278o, and would be in what is called a heliocentric conjunction. Heliocentric (Sun-centered) coordinates uses an overhead view of the solar system with planet location given as degrees of heliocentric longitude. The heliocentric longitude is based on a view from the Sun and is given as the angle between a planet and the vernal equinox. The vernal equinox, 0o, is located within the constellation of Pisces the Fishes, and is the intersection between the ecliptic and the celestial equator.
As the Earth revolves around the Sun it ‘gives’ the Sun its apparent motion eastward along the ecliptic. When the Sun crosses the celestial equator at this intersection it is moving north. At the crossing northern hemisphere winter becomes spring – the opposite seasonal change for the southern hemisphere.
Despite having the same heliocentric longitude, when viewed from the surface of the Earth, the two show an east to west difference of about 2 hours of right ascension.