The planet Neptune will reach Solar Conjunction (i.e. be directly on the other side of the Sun from the Earth) at 1.56 pm GMT on Sunday 4 March 2018. This means that it will both be at its furthest from the Earth this year, about 30.94 AU (30.94 times the average distance between the Earth and the Sun, or about 4 629 000 000 km), and while not completely obscured by the Sun, due to differences in the tilts of the orbits of Earth and Neptune, it will be close enough to the Sun to be invisible for several weeks.
The relative positions of Neptune and Earth at Solar Conjunction (not to scale). Bob's Spaces.
Neptune orbits the Sun at an average distance of 30.11 AU, completing one orbit around the Sun every 165 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Solar Conjunction only four days later each year than the year before, and reaches opposition (when it is directly opposite the Sun seen from the Earth), roughly six months later.
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