Mercury regularly passes between Earth and the Sun as it completes its 88-day orbit, but because its orbit is inclined relative to Earth's, it usually doesn't precisely line up between those two bodies.
While the celestial event is visible from most of Western Europe and North America on Monday 9 May between 12.12pm BST and 7.42pm BST, catching a glimpse of it is no easy task.
The unusual event, which occurs only about 13 times each century, has held a fascination for astronomers dating back to the Renaissance.
Find a telescope or high-powered binoculars fitted with solar filters made of specially coated glass or Mylar.
It'll take about seven and half hours for the planet to make its solar journey, though in California we'll only be able to witness part of that.
If the sky is clear, we'll have a telescope on the lawn of the Pink Palace Museum starting at 10:00 a.m. We'll use solar filters so you can safely observe the Transit.
Experts say watching the transit directly could be unsafe, and one without appropriate eye protection could face troublesome conditions like permanent blindness.
The national space agency will present images of the transit in an hour-long television special, social media mentions, and in photos on their Web site.
It turns out, that's a hot topic these days because scientists are using transits to study exoplanets, or alien worlds orbiting distant stars.
If we put an opaque diaphragm with a small hole on the way of the light rays, then different light rays from each single piece of the observed object (e.g. the solar disc) are cast on different parts of the screen that is placed behind the diaphragm.
The visual and photographic requirements for the transit are identical to those for observing sunspots and partial solar eclipses. First observed in the 1631, the transits were later used to "measure the distance between the Earth and the Sun", NASA said.
Additionally, scientists have found that a transiting planet causes a drop in the sun's brightness.
Last year NASA's Messenger spacecraft slammed into the surface of Mercury, ending a successful 11-year mission.