What causes a total solar eclipse?
The short answer
A total solar eclipse happens when the Moon passes exactly between the Sun and Earth, blocking the Sun's light and casting a shadow onto the planet. It can only occur at new moon. Nothing happens to the Sun itself; it is pure geometry, and it can be predicted to the minute years in advance.
Nothing is wrong with the Sun
This is worth saying plainly, because an eclipse looks alarming. The Sun does not dim, flicker or change in any way. An object simply gets in the way. The Moon orbits Earth roughly once a month, and occasionally that orbit carries it directly across our line of sight to the Sun. The Sun carries on exactly as before; we just cannot see it for a few minutes.
It can only ever happen at new moon
For the Moon to block the Sun, it has to be between us and the Sun. That is the definition of a new moon, the phase when the lit side faces away from Earth and we normally cannot see the Moon at all. So a solar eclipse is the one occasion when the invisible phase of the Moon announces itself, by switching off the sky. A lunar eclipse, by contrast, can only happen at full moon.
A small Moon can cover a huge Sun
The Sun is about 400 times wider than the Moon, which sounds like an impossible mismatch. The Moon wins because it is close. Hold your thumb at arm's length and you can blot out a mountain. How large something looks depends on its real size and its distance together, and the Moon happens to be about 400 times nearer than the Sun. The two factors cancel out and the discs appear almost identical in size.
The shadow is a cone, and only its tip touches Earth
The Moon's shadow narrows to a point. Only that very tip reaches Earth's surface, which is why totality is confined to a narrow strip rather than covering a continent. Surrounding the tip is a much wider region of partial shadow, where the Sun is not blocked completely but has a bite taken out of it. So every total eclipse produces two different experiences: a small area of genuine darkness inside a vast area of partial cover.
The shadow moves faster than sound
The tip of the shadow races across the ground at thousands of kilometres per hour, far quicker than any aircraft. A complete crossing of the planet typically takes only a few hours. You cannot chase it; you position yourself in its path and wait. Because the strip is narrow and lands somewhere different each time, any particular town can wait decades or centuries between total eclipses, even though a total eclipse occurs somewhere on Earth roughly every eighteen months.
Sources
Full transcript: Why the Sun Disappears on August 12
1 The Sun will vanish on August 12th. Here's exactly why. 2 On August the 12th — 18 days from now — the Sun is going to switch off mid-sky. 3 Broad daylight.
Stars will come out. And here is the strangest part: 4 We know the exact minute it will happen. Not a guess — a schedule. 5 Today, exactly why the Sun will vanish, and exactly where you'd have to stand to watch it go.
6 Welcome back to Watt & Why, and welcome to the Eclipse Wave. The countdown is officially on: T-minus 18 days! 7 This is the Sun that vanishes on schedule. First, nothing is wrong with the Sun.
Nothing will happen to the Sun. 8 This is a trick of geometry. The Moon circles us every single month like a clock hand. 9 And on August the 12th, that clock hand sweeps exactly between us and the Sun.
Sun, Moon, Earth — one straight line. 10 The Moon blocks the light and drops its shadow on the Earth. And check the calendar: it can only ever be a New Moon. 11 The invisible one — the Moon nobody's been able to see all month — suddenly announces itself by turning off the sky.
12 Now hold on, you should be suspicious. The Sun is 400 times wider than the Moon. 400! That's not even close to a fair fight.
13 But the Moon has one advantage: it's near. Hold your thumb up close to your eye — it can hide a whole mountain. 14 Small and close beats huge and far. And from where you're standing, Moon and Sun look exactly the same size.
15 Suspiciously exact! How lucky is that fit? That question gets its own episode (Number 7). For now, just enjoy being suspicious.
16 Here's the part almost nobody tells you: the Moon's shadow is a cone narrowing to a point, and only the very tip reaches Earth. 17 So there are two shows on August 12th. Inside the tip: a little dot of night where the Sun disappears completely. 18 And around it: a huge, wide zone of half-shadow where the Sun doesn't vanish — it just gets a bite taken out of it.
19 A dot of night inside a thousand miles of bite. And that dot of night moves! Wednesday, August the 12th — watch it run. 20 It touches down near the top of the world, sprints across Greenland in the afternoon, clips the west coast of Iceland.
21 That's where the show lasts longest: 2 minutes and 18 seconds! 22 Then races down the open Atlantic and slides across northern Spain right as the Sun is setting. Then it's gone out to sea. 23 The whole crossing takes barely an hour and a half, because that shadow is moving faster than sound!
24 You don't chase it — you stand in its way. So where will you be? 25 If you're in that narrow path: night in the afternoon, about 2 minutes of it. 26 Most of Europe and Northwest Africa: a big, dramatic bite out of the Sun.
Northern America (Alaska across to the East Coast): a small nibble around midday. 27 And if you're anywhere else on Earth: the live stream! Honestly, one of the best seats in the house. 28 One warning while we're here: even a 99% bite is NOT night.
Madrid and Barcelona get 99% and stay in glasses the whole time. More on that in a moment. 29 Now, somewhere on Earth gets a total eclipse about every year and a half. Totality is always somewhere, but the dot lands somewhere new each time.
30 And it's small — so any one particular sky can wait decades. Mainland Europe's last turn? 1999. 31 27 years of waiting for August the 12th!
And if clouds win or the path is far away, Spain gets another one just a year later. The clockwork never stops. 32 Before any of us watch anything, one rule starts today: during every partial moment — every bite, every sliver — you NEVER look at the Sun with bare eyes. 33 Not a squint, not a peek.
It doesn't hurt while it happens, and that's exactly what makes it dangerous! 34 Sunglasses are not enough — not even close. Real eclipse glasses, or a projector you can build at home! 35 On Tuesday, we'll show you everything: the glasses, the cereal box trick, all of it.
Rule Number One starts NOW. 36 Here's the plan for the next 18 sunrises: 37 Tuesday: How to watch it safely (the episode to share with every grown-up you know). 38 Thursday: What you'll actually see, minute by minute. 39 Saturday: The eclipse that changed science forever.
40 And then August the 12th — 18 little squares on the calendar. We're going to light up every one of them together. 41 So remember: on August the 12th, a New Moon slides exactly between us and the Sun. Pure clockwork, nothing is wrong.
42 Its shadow is a racing dot of night inside a wide, gentle bite. And from today until then: glasses on, never stare. 43 T-minus 18 days! Tuesday: How to watch the eclipse without hurting your eyes — bring a cereal box.
See you on Watt & Why!
Transcript of the episode, lightly cleaned for readability.
Sample question
Question 1 of 10
What causes a total solar eclipse?
- The Moon passes between the Sun and Earth and blocks the Sun's light correct
- The Sun temporarily stops shining
- Earth's shadow falls on the Sun
- A cloud of dust drifts in front of the Sun
Nothing happens to the Sun itself. An object simply gets in the way.
That is one of 10. The rest are not shown here, and the answer order changes every time you take the quiz.
Written by Watt & Why. Published 21 July 2026. Last reviewed 17 October 2026.