Both of those events will occur this Friday.
Given that there’s a lot of nonsense talked about “Mercury in retrograde,” let’s begin with the closest planet to the Sun.
Mercury in retrograde
When: Thursday, June 18 through Sunday, July 12, 2020
Mercury in retrograde is happening again this month. Note that it’s not physically going to start going backwards in its orbit. Rather, it will happen because Mercury moves faster in its orbit than does Earth. The inner planet will go backwards for a period because it orbits the Sun every 88 days, so its relative position with regard to Earth changes a lot—and so does a line of sight. Think of overtaking a car—that car appears to go backwards relative to you.
Mercury’s retrograde motion is an illusion, it’s nothing to worry about, and it’s very common; it last happened from February 17 to March 10, 2020, and it will happen again from October 14 to November 3, 2020.
You can have a look for yourself at Mercury this week by looking low to the western horizon right after sunset. You will need need binoculars to spot it.
A crescent Venus occulted by a crescent Moon
When: early on Friday, June 19
With Mercury dealt with, let’s get on with Venus, whose occultation by a crescent Moon is much more exciting. A 4%-lit waning gibbous crescent Moon will, for a short time, cover bright planet Venus, which itself will be an 8%-lit crescent.
Venus will vanish behind the Moon’s bright limb and re-appear from behind its darkened limb. However, it will only be technically (not easily) visible from North America and Europe since it will be going on in daylight above the east-northeastern horizon.
It’s therefore best to consider this a chance to see a crescent planet and a crescent Moon in very close proximity—that’s going to be a beautiful pre-dawn sight in itself.
However, if you have a telescope and you’re up for a challenge, read on.
How to see Venus being occulted by the Moon
Firstly, you need to be awake early in the day and be located in the far northeastern and northernmost parts of North America, Greenland, Iceland, the UK, or western, central or northern Europe.
This event is all about watching Venus disappear—ingress—then re-appear—egress—a short period later, though only the latter is visible from some locations.
Find a map
There’s a region-of-visibility map here, followed by predictions for several North America cities and towns (where it will happen really early in the day, but in twilight). For Europe, it’s a breakfast-time event, which means looking for the event in broad daylight.
Get your timings right
The occultation starts from 08:37 BST (07:37 UT) on Friday, June 19, 2020, as seen from the centre of the UK, according to the Sky At Night.
Listen to an expert
“It will be quite difficult to observe, since the 4% sunlit Moon will be 23º from the Sun, and rising at an unfavorable angle to the horizon, so when the Moon/Venus are a few degrees above the horizon, the morning twilight is already very bright,” Dr. David Dunham, president of the International Occultation Timing Association (IOTA), told me.
For Europe and Newfoundland
Use a small “GoTo” telescope and a solar filter. “With the solar filter on, the scope can be pointed at the Sun and then used for a one-object “solar system” alignment,” he said. “That should be good enough for the scope to go to Venus, and then the Moon should be faintly visible.”
For northeastern North America
Dunham recommends using a telescope in shadow (shielded from sunlight) to find Venus, preferably using an automatic “Go-To” system. “In eastern Massachusetts, those who can look out over the sea, can scan with binoculars to find the thin crescent Moon just rising, then look just north of the center of the dark side to see Venus reappear,” he said. “If the sky is very clear, it might be visible naked-eye, but I think just barely.”
The farther northeast you go, the higher the Moon and Venus will be, but the twilight will be much brighter, making it harder to see.
However rare the occultation event is, it’s going to be viewed by most merely as a close conjunction of two beautiful crescents—and that will be good enough for any night sky-watcher after something special to end the week on a high.
Wishing you clear skies and wide eyes.