Make it count
Report a flash
Saw a flash? Email the details below to ak51@hku.hk.
Please include in your email
- Date — UT date.
- Time — UT, to the nearest tenth of a second if possible.
- Location — latitude/longitude + place name.
- Telescope and camera — aperture/type, camera, frame rate.
- Position on the Moon — nearest crater/landmark, or lunar latitude/longitude.
- Frames and duration — frames it appears in; how long it lasted.
- Sky and Moon — phase, cloud, seeing, anything unusual.
- Your name — for follow-up.
- 10-second HD video, attached — 10 seconds around the flash, cut without re-encoding or “enhancing”. Keep the full original untouched and available.
Send to: ak51@hku.hk — with the 10-second HD video attached.
Why it matters
Small rocks, large consequences
- Same debris hits Earth, satellites, and future lunar crews. Old craters alone cannot give today's rate. Flashes measure it as it happens, across the whole Moon.
- Impacts shake the Moon. Flash timings + seismometers probe the interior.
- Each new crater exposes fresh material — a target for orbiters, and one day for surface missions.
- Practical: how often does a patch of ground take a hit of a given size? How much shielding does a habitat need? The flash record answers the first directly, and catches changes — a new stream, a surge — as they happen.
- Two views together: Earth telescopes on the near side + a satellite in lunar orbit at 100–3,000 km, sweeping both sides. Neither alone sees the whole Moon, all the time. Together, they very nearly do — and a flash you record tonight becomes part of that record.
Sources and notes
Where the numbers come from
- Moon Flash archive — NELIOTA 2017–2023 file + per-event frame images. The table reproduces its 103 events.
- NELIOTA system paper (Xilouris et al.) — 1.2 m Kryoneri, twin R/I cameras at 30 fps, 31 flashes in year one, faintest R 11.24, rate 1.96 × 10−7 km−2 h−1.
- NELIOTA project design — goal: magnitude 12 with the 1.2 m telescope.
- Orbit estimate: from ≈7.4 sporadic impacts/hour, whole Moon; counts double per magnitude fainter; only 2–4 magnitudes of gain counted; satellite credited only with the dark-side share in view (Moon radius 1,737 km). Order-of-magnitude estimate, not a measurement.
- Impact Flash citizen-science programme — 4-inch minimum, 25–30 fps video, GPS location, dark-side pointing away from the terminator.
- Lunar Impact Flash portal — upload raw monochrome video + metadata.
- British Astronomical Association guide — aperture, camera, recording advice.
- ALFI detection software — scans monochrome video for flashes.
- Temperatures, masses, rates (1–200 g; 85% at 2,000–4,500 K; 7.4 sporadic, up to 12.6 stream/hour, whole Moon): full NELIOTA validated sample, 192 flashes, 2017–2023.
- Speeds up to 72 km/s; durations of tens of milliseconds; two phases (vapour plume, then cooling ejecta): high-speed flash observations and impact-flash reviews.
Photos: as captioned. Animations, calculator, and phase planner are illustrations/approximations computed in your browser — not recordings; phase dates are approximate.
Your telescope can do this
How an amateur astronomer catches a new flash
- No professional observatory needed. A telescope, a camera that never blinks, and patience.
- The camera watches. A flash may take hours. Check the footage afterwards.
- One confirmed flash counts. Timed to the second, placed on the disc — a real measurement. A second observer elsewhere makes it stronger.
The kit that works
- Telescope: 100 mm (4 inch) or more. Bigger = fainter flashes. Around f/6 or faster frames more dark disc. A tracking mount that holds the Moon centred is close to essential.
- Camera: 25–30 fps or faster. At 30 fps a typical flash spans two frames. Slower, and it can fall between exposures. Monochrome video, uncompressed where possible.
- Accurate time. GPS time, or a network-synced clock, checked. Time to a tenth of a second lets two observers' records be matched.
- Your location. Latitude, longitude, elevation — from a phone GPS.
- Detection software. ALFI, LunarScan, or Flash Detection Software scan hours of video for the brief brightening.
Seestar S50 Pro — bright flashes only
- It can see bright lunar flashes.
- Rough estimate: about magnitude 8–9 or brighter (smaller number = brighter). Fainter flashes will likely be missed.
On the night
- Pick a crescent. Use the planner below. Point at the dark side, away from the terminator. Keep the bright crescent mostly out of frame — its glare hides faint flashes.
- Focus on the dark disc. Earthshine shows the limb and a few markings. Focus on the limb or a bright star first, then reframe. Keep gain/exposure fixed.
- Record, don't stare. Let video run in long segments. Note each file's start time. The flash will be gone before you react — the recording is the evidence.
- Keep the raw files. Do not re-encode, trim, or “enhance” before submitting. Analysis needs untouched frames and timestamps.
- Check every candidate twice. Real: fixed on the surface, ≥2 frames, fades. Cosmic-ray hit: usually 1 frame. Satellite/aircraft: moves. Hot pixel: never moves, repeats nightly.
When the odds are best
- Meteor showers raise the rate — up to about 12.6 per hour, whole Moon, during strong streams. Watch at the shower's peak, on a crescent night.
- Total lunar eclipses: the whole disc is dark, background at its lowest. Any flash, anywhere on the face, is in play.
- Coordinate if you can. Two observers, hundreds of kilometres apart, synced clocks, same dark disc — confirmation that rules out aircraft, satellite glints, and sensor faults. That turns a candidate into a discovery.
Is tonight (or any night) a flash night?
Approximate phase, from the synodic cycle — for planning, not precise ephemeris work. Best: crescents, 4–45% lit. Waxing: evening. Waning: before dawn.
The record so far
103 flashes, mapped and measured
- Who: NELIOTA, 1.2 m telescope, Kryoneri Observatory, Greece.
- How: two fast cameras, 30 frames per second, red + near-infrared at the same time.
- What: 103 events, 2017–2023 — date/time (UT), duration, peak magnitudes, position.
- Select an event to see where it fell.
Loading events…
Where they fell — the near side
Near side, centred on 0° longitude facing Earth. Larger dots = brighter (smaller R magnitude). Positions good to about half a degree.
No event selected. Try the brightest one: 7 July 2019, magnitude 6.65 in red light.
| Date & time (UT) | Duration | Peak R | Peak I | Lat. | Lon. |
|---|
Two events carry a duration range. The full NELIOTA campaign reported 192 validated flashes; the file mirrored here lists 103. Full event pages, with frame images in both bands, are in the linked archive.
Open the Moon Flash archive (event pages and frame images) →
What the record already shows
- Most impactors are small. Over three quarters: 1–200 g, 0.5–3 cm, craters up to about 3.5 m.
- They run hot. 85% peak at 2,000–4,500 K.
- Brighter lasts longer. Longest here: 0.429 s. Shortest: one 33 ms frame.
- Infrared wins. Every event is brighter in I than in R, by roughly 1–2 magnitudes.
How NELIOTA caught them
- Two cameras, one beam-splitter: one per band, 17.0′ × 14.4′ field — a large slice of the dark disc at once.
- Checked by people: software flags candidates in both channels; humans separate impacts from cosmic-ray hits and satellites.
- Limit: about magnitude 12.4 at low lunar phase.
- First year: 31 flashes; 1.96 × 10−7 events per km² per hour over the monitored area.
Scaled to the whole Moon, that rate gives ≈7.4 impacts per hour. A satellite watch would test it directly — both hemispheres, no weather or phase limits.
The view from close up
Why a satellite around the Moon sees more — and dimmer — flashes
- Closer: the same flash looks far brighter — dim ones become visible.
- No air, no weather: nothing blurs or dims it. No night is lost to cloud.
- Far side too: in orbit, it sweeps over the half Earth never sees.
- Every orbit: over the night side again and again — not just on clear crescent nights.
Try it: move the camera closer
Halve the distance: 4× brighter. Drag the slider.
A flash of magnitude 12.0 as seen from Earth would appear as
12.0 magnitude
1× as bright as from Earth
At Earth's distance, at the edge of ground detection (about magnitude 12.4). Most this faint go unrecorded.
From the ground — near side only. The far side is never seen, and every flash must cross 384,400 km plus our atmosphere.
| From the ground | Low orbit · 100 km | High orbit · 3,000 km | |
|---|---|---|---|
| Distance | 384,400 km | 100 km | 3,000 km |
| Brighter | — | ~15 million × | ~16,000 × |
| Sees | Near side only | Both sides, in turn; small patch at once | Both sides, in turn; wide patch at once |
| Air & weather | Air blurs and dims; cloud ends the night | None | None |
| When | Clear nights, near crescent phase | Every orbit, over the night side | Every orbit, over the night side |
Brightness gains use the inverse-square law: factor = (384,400 / d)². Close up, a small camera can out-see a large one far away.
How many flashes would one satellite see? — an estimate
Estimate, not a measurement. One satellite, one wide camera, always recording the night side.
| Altitude | Brighter than from Earth | Flashes seen |
|---|---|---|
| 100 km — low orbit | ~15 million × | 5–20 a day · ≈2,000–7,000 a year |
| 1,000 km | ~150,000 × | 30–130 a day · ≈11,000–48,000 a year |
| 3,000 km — high orbit | ~16,000 × | 60–230 a day · ≈22,000–83,000 a year |
- Start: ≈7.4 flashes an hour across the whole Moon — ≈90 a day on the dark half.
- Fainter = more common: counts roughly double per magnitude fainter. Only 2–4 magnitudes of gain are counted; the rest is lost to background glow, very short flashes, and a small camera.
- Coverage limits it: at any moment, the satellite sees only part of the dark side — ≈1% of the Moon at 100 km, ≈16% at 3,000 km. Higher orbit wins by seeing more at once.
- Trade-off: low orbit sees the dimmest flashes; high orbit sees more sky. A narrower camera sees fewer, in proportion to its view.
- Showers: up to ≈1.7× more during a strong stream (12.6 an hour, whole Moon).
Why dimmer matters
- Faint flashes far outnumber bright ones — small rocks are far more common.
- Gram-scale impactors become visible, not just kilogram-scale ones.
- The rate sets the hazard for crews and habitats on the surface.
- It calibrates crater dating — surfaces are dated by counting craters.
- Two colours give temperature — free of air distortion.
The half we never see
The far side is not dark. It is just turned away.
- Same face, always: the Moon rotates once per orbit, so the far side never turns toward Earth.
- Not dark: half of each month in sunlight. Flashes show only on ground in darkness.
- Half the Moon, unmonitored: no Earth telescope has ever watched it for flashes.
- Orbit fixes that: a satellite sweeps both sides in turn.
- Both sides count: streams hit the two sides at different rates through the year. Only both views give the true rate.
What a Moon flash is
A tenth of a second of geology, in light
See a flash happen
A rock strikes the dark side. Slowed down here — in real life the flash is over in less than a tenth of a second.
- 1 · A rock arrivesSmall, moving at tens of kilometres per second.
- 2 · It flashesThe ground glows white-hot, briefly.
- 3 · It fadesThe glow cools. A small new crater is left.
Illustration, not a recording. The approach is shortened. Real time shows why video, not eyesight, is needed.
- No air. Even a pebble-sized rock arrives at full speed — commonly 20–70 km/s.
- Energy in a small patch. It is dumped into ground no wider than a room, in far less than a second.
- Only a little becomes light. Most energy digs the crater and throws out debris. The light is what we can see from 384,400 km away.
- Light tells a story. Brightness, fade speed, and colour change point to the rock's mass, speed, and energy.
- Strike. Rock at the contact point is shocked, melted, and vaporised almost instantly.
- Plume. A hot vapour plume glows — the sharp rise. About 2,000–4,500 K.
- Cooling glow. Molten droplets cool and dim over tens of milliseconds, sometimes with a longer, fainter tail.
Why flashes look redder than white-hot. The same flash is almost always brighter in near-infrared I than in red R — e.g. 6.65 in R vs 5.49 in I. Smaller magnitudes are brighter.
Two colours at once give the temperature — and help rule out sensor artefacts, which would not behave the same way in both channels.
Watch a flash — slowed down so you can see it
Illustration from a typical light curve: fast rise, then a fading tail. At real speed, it is over before you can blink.
Simulated crescent view. Not a recording.
Press Play flash. A 66 ms flash spans about 2 frames at 30 fps — too fast for eyesight. Video catches it.
Only the dark side shows them
- Bright side: hidden. A flash is faint next to sunlit ground. It is drowned in glare.
- Dark side: visible. The only background is earthshine — sunlight bounced off Earth. The flash stands out.
- Watch away from the day–night line. Keep the bright crescent mostly out of frame.
- Best time: crescent phases. A few days each month, when the dark disc is wide and the Moon is up at night.
- Bonus: total eclipse. The whole disc goes dark at once. In January 2019, a flash during totality was seen with the unaided eye.
Reading the magnitudes in this page
- Smaller = brighter. Each step of 1 magnitude is about 2.5× brighter. 5 magnitudes = 100×.
- Example: magnitude 6.7 is roughly 25× brighter than magnitude 10.2.
- Faintest first-year flash: magnitude 11.24 in red light — about 2 magnitudes fainter than anything recorded before it.
Magnitudes here are as seen from Earth, unless another distance is stated. Moving closer changes them — see the next page.
The short version
- A rock hits. It strikes at tens of kilometres per second. No air slows it down.
- It flashes. The ground glows white-hot — usually for less than a tenth of a second.
- We watch. Earth telescopes see the near side only. A camera close to the Moon could see far more — fainter flashes, and the far side too.
Start here
What is a Moon flash?
What happens in that tenth of a second — and how brightness is measured.
Read more →Why orbit sees more
Closer, no weather, no air — and a view of the far side. Try the distance slider.
Read more →103 flashes, mapped
103 flashes from 2017 to 2023, on a map of the Moon.
See the archive →Catch one yourself
A telescope, a video camera, and a crescent Moon.
How to observe →Report a flash
Email the details, attach 10 seconds of HD video, to ak51@hku.hk.
Build a report →No telescope? Start with What is a Moon flash? — about two minutes.