The Moon passes right in front of a star or planet, making it disappear for a little while. This is called a lunar occultation. They happen pretty often, though most people wouldn’t notice. Unlike eclipses, which get all the attention, occultations are quick and subtle. Here’s what you need to know.
What are Lunar Occultations? (And the Difference Between Transits and Eclipses)
A lunar occultation happens when the Moon moves in front of a star or planet, blocking it from view for a short time. Since the Moon is much closer to us than most other objects in space, it looks large enough to completely cover them, at least from our perspective on Earth.
This isn’t to be confused with transits and eclipses, though. These lunar events are similar but different.
- Occultation: This is when one object in space fully blocks another. In a lunar occultation, the Moon completely covers a star or planet as it moves across the sky. The object seems to vanish instantly and then suddenly reappear.
- Transit: A smaller object moves across a larger one without covering it entirely. For example, when Mercury or Venus passes in front of the Sun, they look like tiny dots slowly drifting across its surface. You might also see Jupiter’s moons transit across the planet, casting small shadows.
- Eclipse: This happens when an object blocks light from another, creating a shadow. A solar eclipse is when the Moon moves in front of the Sun, casting a shadow on Earth. A lunar eclipse is when Earth blocks sunlight from reaching the Moon, making it appear dark or reddish.
Lunar occultations aren’t as famous as eclipses, but they happen more often and are really useful for astronomers. Watching how a star disappears and reappears can help refine measurements of the Moon’s orbit and reveal details about distant stars.
Lunar occultations even led to the discovery of binary star systems!
The 3 Types of Lunar Occultations
Not all lunar occultations are the same. The Moon can pass in front of different types of objects, and the way it happens depends on what’s being covered. Some are common, some aren’t. All of them offer something interesting to observe, though!
Lunar Occultations of Stars
Sometimes, the Moon passes in front of a bright star, blocking it from view for a short time. This happens pretty often, especially with stars near the ecliptic, the path the Sun appears to follow in the sky. Some of the most commonly occulted stars are Aldebaran, Regulus, Spica, and Antares.
What makes this interesting is that the star doesn’t fade out gradually. It just vanishes in an instant. That’s because the Moon has no atmosphere to scatter its light. If there were an atmosphere, the star would slowly dim before disappearing, kind of like how the Sun fades during a sunset.
Astronomers use these events to study stars in detail. By timing exactly when a star disappears and reappears, they can measure its size and sometimes even discover hidden companion stars. It’s a pretty simple way to learn more about the universe.
Lunar Occultations of Planets
Unlike stars, planets don’t get occulted by the Moon as often, but when they do, it’s a pretty amazing sight. Since planets are much closer to us than stars, they appear as actual disks rather than tiny points of light. Pretty cool to watch.
Venus is one of the easiest planets to see during an occultation because it’s so bright. Even in daylight, you can spot it with a telescope. Jupiter is another great one, since you can often see its moons and cloud bands just before it disappears behind the Moon.
When Mars gets occulted, it sometimes happens around the time of its opposition, meaning it looks bigger and brighter than usual. And then there’s Saturn. Watching its rings slowly slip behind the Moon makes for a really stunning view.
As with stars, occultations of planets have value to astronomers by helping them refine planetary positions and study their movements.
Related
Grazing Occultations
A grazing occultation happens when a star or planet barely skims along the edge of the Moon instead of disappearing behind it completely. As it moves along the Moon’s uneven surface, it might flicker in and out of view, popping in and out behind lunar mountains and valleys.
These events are especially interesting because they give us a closer look at the Moon’s topography. Since the Moon isn’t perfectly smooth, astronomers can use grazing occultations to map out its peaks and dips with pretty impressive accuracy.
The tricky part is that you have to be in just the right spot to see one. A slight shift in location can mean the difference between catching the object flickering along the edge or missing the event entirely. That makes grazing occultations rare and a bit of a challenge to observe, so consider yourself lucky if you get to experience one!
Occultations Were Used to Determine Ephemeris Time Before Atomic Clocks
Before atomic clocks, astronomers had to get creative when it came to keeping precise time. One way they did this was by using lunar occultations, timing exactly when a star disappeared behind the Moon and when it reappeared.
Since the Moon moves in a predictable way, these observations helped track small changes in the Earth’s rotation and led to the development of Ephemeris Time (ET), a system based on the motion of celestial objects rather than the Earth’s inconsistent spin.
For a while, Ephemeris Time was the best way to maintain an accurate time standard. But over time, scientists realized that Earth’s rotation isn’t as steady as they once thought. It speeds up and slows down slightly due to things like gravitational forces and climate shifts.
This made it clear that a better system was needed. When atomic clocks came along, they provided a much more reliable way to measure time. This lead to the adoption of Terrestrial Time (TT), which is what we use today.
How to Observe (and Record) a Lunar Occultation
You don’t need a fancy telescope or expensive gear to watch a lunar occultation. A decent pair of binoculars or a small telescope will do the trick. The best time to catch one is when the Moon is waxing (growing), because the dark edge moves in first which makes it easier to see a star or planet suddenly vanish. If the Moon is waning (shrinking), the bright side leads, so it’s more difficult to spot fainter stars disappearing.
If you want to record an occultation for science, you’ll need to measure the exact moment the star or planet disappears behind the Moon and when it reappears. This helps astronomers fine-tune our understanding of the Moon’s orbit and improve star position data.
To get an accurate time reading, use a stopwatch, a time-syncing app, or even a GPS-based clock. Some people even record the event with a video camera that includes a time stamp. This helps with precision.
Once you’ve got your data, you can submit it to the International Occultation Timing Association (IOTA), where it actually contributes to real scientific research. Pretty cool for something you can do from your backyard!
Lunar occultations may not get as much hype as eclipses, but they’re just as cool. Watching a star or planet suddenly disappear behind the Moon is a great reminder that space is always moving, even if we don’t always notice.
