How to See an Astronomical Transit: Your Complete Guide

Ever been looking at the Sun (safely, I hope!) or a planet like Jupiter and spotted a tiny, perfect black dot creeping across its face? If you have, you’ve seen it. One of the most elegant and fascinating things in all of astronomy.

It’s called a transit.

It’s a moment where you can literally see the solar system ticking along like a giant clock. A cosmic alignment that feels impossibly huge and surprisingly personal, all at once.

But it’s not an eclipse. It’s something far more subtle.

An astronomical transit is just the passage of one celestial body directly between a larger one and you, the observer. From our little rock, that usually means we’re seeing a planet (Mercury or Venus) cross the disk of the Sun. Or, it could be a moon crossing the face of its home planet. This guide is your complete map. We’re going to cover exactly how to see an astronomical transit, from the gear you need to the safety steps you absolutely, positively cannot skip.

More in The Observer’s Sky Category

Measuring Star Distance with Parallax

How to See a Celestial Occultation

Key Takeaways

Before we dive deep, here’s the need-to-know info to get you started.

• It’s an Alignment: Simple as that. One object passes in front of another from your point of view. The most famous are Mercury and Venus crossing the Sun.
• Solar Safety Is Not a Suggestion: I’m serious. Never look at the Sun without a certified solar filter made for your telescope or binoculars. You can cause permanent, life-altering eye damage in seconds.
• There Are Different Kinds: You can hunt for rare planetary transits. You can try to catch fast-moving satellite transits (like the ISS). Or, you can watch the nightly show of moons transiting Jupiter, which happens all the time.
• You Must Plan: You can’t just walk outside and hope to see one. These events are predicted down to the second. A good astronomy app or transit calculator is your best friend.

So, What Exactly Am I Looking For?

Right, let’s clear this up. People hear “transit” and “eclipse” and instantly jam them together in their minds. They’re related, sure, but they are not the same thing.

A solar eclipse happens when our Moon passes in front of the Sun. Because of a wild cosmic coincidence, the Moon is just the right size and distance to block the entire Sun.

A transit is more delicate. Much more.

When Mercury or Venus transits the Sun, they look like a tiny, sharp, perfectly round black dot. They don’t block the Sun’s light in any way you’d notice. They just create a tiny silhouette. Mercury is incredibly small, just 1/194th of the Sun’s diameter. Venus is bigger, about 1/32nd of the Sun’s width, but even then, it’s just a small spot.

What you’re looking for is the ultimate proof of a clockwork solar system. You’re watching a planet—a whole other world—moving in its orbit. For a few precious hours, that orbit crosses our line of sight with the Sun. It’s a profound, visual “Oh, wow” moment that reminds you we live in a dynamic, 3D system.

Are All Astronomical Transits the Same?

Not even close. “Transit” is a big-tent word, and the sky offers a fantastic variety show. The one you choose to hunt down really depends on your patience, your gear, and sometimes, just plain old luck.

The “Big Ones”: Why Are Transits of Mercury and Venus So Rare?

Good question. You’re probably thinking, “If Mercury and Venus orbit between us and the Sun, shouldn’t we see them transit all the time?” It makes perfect sense. The answer? It’s all about tilt.

The orbits of Mercury, Venus, and Earth are not on the same perfect, flat plane. Imagine them as three Hula-Hoops, one inside the other, but all tilted slightly. Because of this tilt, most of the time Mercury and Venus pass above or below the Sun from our perspective, even when they’re technically “in front.”

A transit only happens during those rare, special moments when Earth, the planet, and the Sun line up perfectly in all three dimensions. For Mercury, this happens about 13 or 14 times a century. The last one was in 2019. The next is in 2032.

For Venus? It’s far, far rarer. Transits of Venus happen in pairs separated by eight years, but those pairs are separated by more than a century. The last pair was in 2004 and 2012. The next one isn’t until 2117.

The ‘Blink and You’ll Miss It’ Show: What About ISS Transits?

Want to see a transit this week? You probably can. You just have to be quick. Really, really quick.

The International Space Station (ISS) zips around the Earth every 90 minutes, screaming along at over 17,000 mph. Several times a month, its orbital path will line up to make it cross the Sun or the Moon from someone’s perspective. The challenge is that the path of visibility on the ground is razor-thin, often just a few miles wide.

What’s more, the transit itself is shockingly fast. An ISS solar or lunar transit lasts for… about one second. Literally.

It’s the very definition of “blink and you’ll miss it.” But catching that perfect, H-shaped silhouette zipping across the Moon’s craters or a sunspot is one of the biggest thrills in amateur astronomy. It just requires pinpoint planning.

Jupiter’s Game of Shadows: Can I See Moons Transit Other Planets?

Yes! And this is, hands down, one of my favorite things to watch. It’s one of the most rewarding and accessible ways to see a transit. Jupiter’s four big Galilean moons (Io, Europa, Ganymede, and Callisto) orbit the giant planet on a plane we see almost perfectly edge-on. This means on almost any given night, one of them is doing something interesting: disappearing behind Jupiter (an occultation), passing into its shadow (an eclipse), or transiting across its cloudy face.

When a Jovian moon transits, you can often see two things with a modest telescope:

1. The moon itself: A small, bright-ish dot moving across the planet’s cloud bands.
2. The moon’s shadow: A distinct, pitch-black, circular dot that follows or leads the moon.

Seeing both the moon and its shadow on the face of another world… that’s an experience that just never gets old.

The Ultimate Challenge: Can Amateurs Really Detect Exoplanets?

This is the big one. For decades, this was pure sci-fi. Today, skilled amateurs can and do detect planets orbiting other stars.

How? They use the “transit method,” the very same one NASA’s TESS satellite uses. This means pointing a telescope with a super-sensitive camera (a CCD or astronomy-grade CMOS) at a star known to have a transiting planet. They then take hundreds of pictures over several hours and measure the star’s brightness in every single one. When the exoplanet passes in front of its star, it blocks a tiny fraction of the starlight—maybe 1% or less. This causes a measurable “dip” in the star’s light curve.

This is advanced work, all about the data. You aren’t “seeing” a dot. You are “measuring” a shadow. But in doing so, you are confirming the existence of a world hundreds of light-years away from your backyard.

Is Seeing a Transit Dangerous? (The Big Safety Talk)

This section is the most important one in this entire article. Read it. Read it twice. No, three times.

When we talk about transits of Mercury, Venus, or the ISS, we are talking about solar transits. This means you will be pointing your equipment directly at the Sun.

You can blind yourself. Permanently.

I’m not being dramatic. I’m being 100% literal.

Why Can’t I Just Stare at the Sun?

Staring at the Sun, even for a few seconds, focuses a firehose of intense, unfiltered light and radiation (including infrared and ultraviolet) onto your retina. This light literally cooks the cells. It’s called solar retinopathy.

And here’s the truly terrifying part: it doesn’t hurt. You will not feel it happening. The damage is painless and, in many cases, irreversible.

This is why you must use a proper filter.

Let’s be brutally clear about what WILL NOT work and will land you in the hospital:

• Sunglasses (I don’t care if you stack 10 pairs. No.)
• Smoked glass
• Exposed film negatives
• Welding glass (unless it is specifically shade #14, and only for looking with your naked eye, never with a telescope)
• Mylar-like food wrappers
• Any “sun filter” that screws into the eyepiece of a telescope. These are cheap, dangerous, and known to crack from the heat. If you have one, throw it away. Right now.

What is a “Safe Solar Filter” and How Do I Use It?

A safe solar filter is one that is specifically designed for astronomy and meets the ISO 12312-2 international safety standard. These filters block 99.999% of the Sun’s visible light and 100% of its harmful IR and UV radiation.

They come in two main types:

1. White-Light Filters: These are the most common. They’re made from a special black polymer (like from Thousand Oaks Optical) or a coated Mylar-like film (like Baader AstroSolar film). They show the Sun in a neutral white or a pale orange/yellow.
2. Hydrogen-Alpha (H-alpha) Filters: These are specialized, expensive telescopes or filters (like from Lunt or Coronado) that only let a very narrow wavelength of red light through. They reveal the Sun’s active atmosphere, showing solar flares, prominences, and filaments.

For a transit, a white-light filter is perfect.

Here’s the golden rule: The filter always goes on the front of the telescope, binoculars, or camera lens. It must cover the entire aperture, blocking the light before it even enters the instrument. Secure it with tape so the wind can’t blow it off. Before every single use, hold the filter up to a bright light (not the sun) and check it for any pinholes, scratches, or tears. If it’s damaged, throw it out.

What’s This “Projection Method” I’ve Heard About?

There is one safe way to view a solar transit without a filter: solar projection.

This method uses your telescope or one side of your binoculars as a projector. You point the instrument at the Sun (again, never look through it!) and aim the eyepiece at a piece of white cardboard held about a foot away. You’ll see a bright, projected image of the Sun appear on the card. You can focus the eyepiece until the Sun’s edge is sharp.

This is a great way to show a transit to a group of people, since everyone can look at the card at once. The downside? The telescope’s insides will get very hot. This method isn’t recommended for expensive, complex telescopes with plastic parts, as the heat can melt them. But for a simple refractor or Dobsonian, it works great for short periods.

What Gear Do I Actually Need to See a Transit?

Your equipment needs will vary wildly. It all depends on your target.

Can I See a Transit With Just My Eyes?

For most transits, no. Mercury? Forget it. It’s way too small to see against the Sun without magnification.

The rare exception is a transit of Venus. Venus is large enough to be spotted as a tiny speck with the naked eye, if you are wearing certified-safe solar eclipse glasses (which meet the same ISO 12312-2 standard).

You can also sometimes spot the ISS with the naked eye if it transits the Moon, but you have to know exactly when and where to look. It will be a tiny, fast-moving dot for a split second.

Are Binoculars a Good Starting Point?

Yes! Binoculars are a fantastic tool for transits, provided you have the right filters. You must buy a pair of white-light solar filters that are designed to fit securely over the front of both binocular lenses.

With filtered binoculars, you can easily spot Venus transiting the Sun, as well as large sunspot groups. A transit of Mercury will be a challenge, but on a steady tripod, it’s possible. Binoculars are also my favorite tool for just watching Jupiter. You can’t see the transits themselves, but you can clearly see the four Galilean moons as tiny pinpricks of light huddled next to the planet.

What Kind of Telescope Works Best?

Any good-quality telescope will show you an astronomical transit. A small 60-80mm refractor telescope with a good solar filter will provide a sharp, clear view of a Mercury transit. A 6-inch or 8-inch Dobsonian reflector will also give a fantastic view, though you will need a full-aperture solar filter, which can be more expensive.

For Jupiter’s moons, magnification is key. You’ll want a telescope that can comfortably give you 100x to 150x magnification. This will cleanly separate the moons from the planet’s glare and make the transits and their shadows crystal clear.

For exoplanets, you’re in a different league. You’ll need at least an 8-inch telescope on a solid, clock-driven equatorial mount that can track a star perfectly for hours, plus a dedicated astronomy camera.

How Do I Find Out When a Transit is Happening?

You’ve got the gear. You know the safety rules. Now for the most important part: the hunt. Transits are all about timing.

Where Can I Get Reliable Transit Predictions?

For major events like planetary transits, the big astronomy players are your best bet.

• NASA’s Eclipse Web Site: This is the gold standard for all eclipse and transit data. This NASA Transit Page is an excellent, high-authority resource for an ephemeris of Mercury and Venus transits.
• Sky & Telescope and Astronomy magazine websites: They will have detailed articles, maps, and guides months in advance of a major transit.
• Stellarium: This free desktop planetarium software is essential. You can plug in any date and time to see where objects are.
• Mobile Apps: Apps like SkySafari, Star Walk, or PhotoPills are invaluable for planning in the field.

For Jupiter’s moon transits, Sky & Telescope has an excellent online tool, and apps like SkySafari will show you the moons’ positions in real-time or for any future date.

How Do I Use a Transit Calculator for the ISS?

This is a special case, so pay attention. Because the visibility path is so narrow, a general forecast isn’t good enough. You need a calculator that knows your exact GPS coordinates.

The best-known tool is Transit-Finder.com. It’s a game-changer.

Here’s the drill: You go to the site and either drop a pin on your exact observing location or enter your latitude and longitude. You specify a date range (say, the next 7 days). Then you hit “Calculate.”

The tool does its magic. It will check the ISS’s orbit against your location and show you a list of all upcoming solar and lunar transits visible from your spot. It gives you a map of the visibility path and a precise time (e.g., 14:32:15.5 EST).

Your job is to be set up, filtered, focused, and waiting at that exact second.

Okay, I’m Ready. Give Me the Step-by-Step for a Solar Transit.

Let’s walk through it. Imagine a transit of Mercury is happening tomorrow.

Step 1: The Pre-Game (Days Before)

• Know Your Times: Know the exact “contact times.” These are: First Contact (when the planet’s disk first “touches” the Sun’s edge), Second Contact (when it’s fully on the Sun), Third Contact (when it reaches the opposite edge), and Fourth Contact (when it’s completely off).
• Check Your Gear: Is your solar filter pristine? No, I mean pristine. No scratches, no holes. If you’re using a film filter, is it securely attached to its cell?
• Practice: If you haven’t used your solar setup in a while, practice finding the Sun safely during the day. Don’t let the big moment be your first time. You’ll be fumbling and stressed. Trust me.

Step 2: The Setup (Day Of)

• Get Out Early: Give yourself at least 30-60 minutes before First Contact. You don’t want to be scrambling and rushing.
• Align Your Finder: Do not use a standard finder scope to find the Sun. You’ll melt it and/or blind yourself. The safest way is to put the main filter on the telescope, then cap the finder scope or attach a solar filter to it as well.
• Attach the Main Filter: Place the solar filter on the front of the telescope. Secure it with masking tape or painter’s tape, ensuring it cannot be dislodged by a gust of wind.
• Find the Sun: Point the telescope in the Sun’s general direction. Look at the shadow the telescope casts on the ground. Fiddle with the scope’s position until its shadow is as small and compact as possible. That means you’re pointing directly at the Sun.
• Focus: Look through the eyepiece (at low power). You should see a bright, white or yellow-orange disk. Focus the telescope on the edge (the “limb”) of the Sun until it is perfectly sharp. If there are any sunspots, they make excellent focusing targets.

Step 3: The Main Event (Watching the Transit)

• Wait: A few minutes before First Contact, keep your eye on the exact spot on the Sun’s limb where the transit is predicted to begin.
• Look for the “Notch”: The very first sign will be a tiny, perfectly round “notch” appearing on the Sun’s edge. That’s it. First Contact.
• Patience: A planetary transit is a slow, majestic event. It takes hours to cross the Sun. Mercury or Venus will appear as a perfectly black, perfectly round circle. It will look completely different from a sunspot, which is irregular in shape and has a grayish “fuzzy” edge (the penumbra).
• Take Breaks: Your eye will get fatigued. Take frequent breaks to rest.
• Enjoy the Egress: Don’t pack up early. Make sure to watch for Third and Fourth Contact as the planet slowly leaves the Sun’s disk.

What About Transits Not Involving the Sun?

Right, let’s talk about my favorite: Jupiter. This is the perfect event for a beginner with a new telescope.

How Can I Watch Jupiter’s Moons Play Tag?

This is just so much fun. And the best news? No solar filter needed! None. You’re observing at night, and Jupiter is perfectly safe to view.

First, check your predictions. Use an app like SkySafari to see what’s happening with Jupiter tonight. The app will have a little diagram of the moons. It might say, “Io Transit” or “Ganymede Shadow Transit.”

Next, find Jupiter. It’s one of the brightest “stars” in the night sky. You can’t miss it.

Now, use your telescope. Center Jupiter in your low-power eyepiece, then switch to a higher-power one (100x or more). You’ll see the planet as a small, bright disk, possibly with its two main cloud belts visible. You’ll also see its four Galilean moons as tiny stars lined up beside it.

If a transit is in progress, look on the face of Jupiter. You might see a tiny dot, the moon itself. It can be hard to see, as it’s bright against a bright background. But the real prize is the shadow. The moon’s shadow will be a small, jet-black, perfectly round dot on Jupiter’s clouds. It’s an unmistakable sign that you are watching a transit.

Because Jupiter and its moons are moving, you can watch the shadow creep across the face of the planet over the course of an hour or two. It’s a dynamic, real-time event that is just deeply cool to see.

I Want to Go Deeper. How Do I Photograph or Record a Transit?

Witnessing a transit is one thing. Capturing it is the next level.

What’s the Easiest Way to Get a Photo?

It’s already in your pocket. Smartphone “afocal” astrophotography (literally, holding your phone’s camera lens up to the telescope’s eyepiece) has gotten incredibly good. Get a simple, cheap smartphone adapter that clamps your phone to the eyepiece. This holds it steady.

For a solar transit, this is the way to get a great shot. Your telescope must have its solar filter on. You’ll be able to snap a photo showing the Sun’s disk and the tiny black dot of the transiting planet.

This method also works spectacularly for ISS lunar transits. Set your phone to record high-frame-rate video (slow-motion mode) and just hope you catch the one-second-long event.

What Do I Need for “Serious” Astrophotography?

If you want those razor-sharp, detailed images, you’ll need to go a bit further.

• Camera: A DSLR or mirrorless camera attached to the telescope’s focuser (this is called “prime focus”), or a dedicated, high-speed planetary astronomy camera from brands like ZWO or QHY.
• Tracking Mount: For a long-duration solar transit, you need a mount that tracks the Sun. For exoplanets, this is non-negotiable.
• Video is Key: The best planetary and solar images are not single shots. Astrophotographers capture thousands of frames in a high-speed video file.
• Software: They then use free software like AutoStakkert! to analyze the video, throw out the blurry frames (caused by air turbulence), and “stack” the best 10% on top of each other. Finally, they use a program like Registax to sharpen the details.

This “lucky imaging” technique is how you get from a wobbly image to a stunning, detailed portrait of a planet or a slice of the Sun.

What Makes a Transit So Scientifically Important?

When you watch a transit, you’re not just enjoying a pretty sight. You’re participating in an observation that literally defined our place in the universe.

How Did Old-Time Astronomers Use Transits?

Back in the 18th and 19th centuries, the “Holy Grail” of astronomy was figuring out the exact distance from the Earth to the Sun. This distance, the “Astronomical Unit” (AU), was the yardstick for the entire solar system. But nobody could measure it directly.

Astronomers, including Edmund Halley, realized that if they could precisely time a transit of Venus from different, widely-separated locations on Earth, they could use the principles of parallax (the same effect that makes your thumb “jump” when you close one eye and then the other) to calculate the AU.

This quest led to massive, globe-spanning scientific expeditions. It’s why Captain James Cook went on his famous first voyage to Tahiti in 1769—to observe the Venus transit. These heroic, difficult, and often-failed observations were the first-ever international scientific collaborations, and they eventually gave us the answer that forms the basis of all modern astronomy.

How Do Transits Help Us Find New Worlds?

Today, the transit method is having a massive comeback. It is the single-most productive method we have for finding planets orbiting other stars (exoplanets).

When a planet passes in front of its star, it blocks a tiny, tiny amount of light. Space telescopes like NASA’s Kepler and TESS are designed to stare at hundreds of thousands of stars at once, just looking for these periodic, minuscule dips in brightness.

The depth of that “dip” tells astronomers how big the planet is compared to its star. The time between the dips tells them the planet’s “year”—how long it takes to orbit. This simple, elegant method has revealed thousands of new worlds, from fiery-hot “Hot Jupiters” to small, rocky worlds that might even be habitable.

Why Bother? What’s the Real Magic of a Transit?

Look, a transit of Mercury or Venus lasts for hours. An ISS transit is over in a second. A transit of Io’s shadow across Jupiter is a patient, nightly affair.

So why do we do it? Why go through all the planning, the safety checks, and the waiting?

Because a transit connects you to the cosmos in a way nothing else can. It’s not a static photo in a textbook. It’s real. It’s happening now. That tiny black dot isn’t a speck of dust on your lens.

It’s a world.

A world with its own geography, its own history, and its own path through space. And for a brief moment, its path has crossed yours.

Watching a transit gives you a visceral, gut-level understanding of scale, motion, and time. It shrinks the solar system down to something you can see and comprehend, all while reminding you of its true, mind-boggling immensity.

The next transit is waiting. You just have to know where to look.

FAQ – How to See an Astronomical Transit