Sheet 4,092 — the night M33 came back
A 21.78 reading, the best of the season, and the one object that tells you whether a sky is genuinely dark or only feels like it.
The sheet was opened at 21:04 with the meter reading 21.61 and closed at 04:12 with it at 21.78 — the best number of the season and the fourth best since the lighting on the valley road was replaced. Between those two readings, eleven people stood on the lawn and found M33 without being told where it was.
That is worth writing down, because M33 is the object this site is actually measured by.
Why it is the test
The Triangulum Galaxy is large, close and faint in a particular way: its light is spread across an area bigger than the full moon, so almost none of it is anywhere at once. Aperture does not rescue it. A 610 mm mirror on a poor sky shows you a slightly brighter patch of poor sky. What shows it is the sky itself being dark.
The question the warden asks
Can you see it without being shown where it is?
Being shown where it is contaminates the answer — the eye is very willing to find a smudge it has been promised. So the naked-eye guide walks people to the Andromeda–Triangulum region, names nothing, and waits.
| 21.78 | mag/arcsec², sky quality at 02:40, meter facing zenith |
| 11 of 14 | found it unaided within four minutes |
| 0 of 14 | found it in the first ninety seconds, before dark adaptation |
The second and third figures are the interesting pair. Nobody sees M33 in the first ninety seconds. The eye needs about twenty minutes to come up, and the last part of that curve is the part that matters — which is why the programme puts the naked-eye half hour first and the 610 second, against every instinct a visitor arrives with.
What we did with the rest of it
The order taken
21:04 to 04:12
- Lawn, unaided. Summer Triangle, the Milky Way through Cygnus, then the Andromeda region with nothing named.
- 355 at 71×. M31 with both dust lanes, then M33 for the people who had not got it unaided.
- 610 at 287×. M15 first — a globular is the right thing to put in front of somebody who has spent an hour on faint smudges.
- 610 at 542×. Two doubles, while the seeing held at 2/5.
- Imaging bench. Twenty minutes on NGC 7331 for the two who had brought cameras.
✓ How it is done here
The 542× eyepiece went in at 01:20 and came out at 01:26, because the seeing softened. Nothing was said about it beforehand and everybody noticed. The fastest way to teach what seeing means is to let a good view get worse while somebody is looking at it.
✕ Where the night went wrong
The imaging bench lost forty minutes
The east pier’s polar alignment had drifted after the June resurfacing and nobody had checked it, so the first two stacks trailed and were binned. That is a maintenance failure, not a weather one, and it is on the sheet as such.
If you are coming up for this M33 is an autumn object from here. In July it does not clear the trees until after midnight, so a public night starting at 20:30 will not get it — the deep-sky session will.
What the reading actually means
A sky-quality meter reads the brightness of the background in magnitudes per square arcsecond. Bigger is darker, and the scale is logarithmic, so the difference between a good suburban sky at 20.0 and this one at 21.78 is not nine per cent — it is a factor of about five in background brightness.
Before quoting a reading
What has to be true
- Meter at the zenith, not at an angle
- Moon down, and down for at least an hour
- No aircraft or satellite in the cone
- Three readings, the median taken
- Air temperature logged — the meter drifts with it
We publish the number at the top of every sheet because a number is harder to argue with than a feeling, and because the direction it moves is the whole point: it has gone 0.31 magnitudes the wrong way in eleven years.
Read against
- Site sky-quality log, sheets 3,004 onward, meter readings at zenith
- Instrument maintenance record, east pier, June resurfacing entry
- Naked-eye recovery counts, guides’ tally sheets, this season
What to do about it
The next new-moon week is the one to book
Deep-sky sessions run to dawn and go out on the 610. Autumn fills first.