BLACKETT OBSERVATORY
Marlborough College
Welcome to the Marlborough College Blackett Observatory, home to the largest refracting telescope in Wiltshire. The Observatory is a key facility for the study of Astronomy by pupils at Marlborough College. It is open to the public through the Friends of the Marlborough Telescope membership scheme. School and Society visits are welcome, please contact the Director to arrange your visit.
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What's Up!
Week of 21st September 2026
- Astronomical Twilight ends at 21:01 BST at the start of the week and at 20:46 BST by the end of the week.
- The September Equinox occurs at 01:08 BST on Wednesday, giving Earth equal length day and night. This marks the start of astronomical autumn for us in the northern hemisphere.
- Neptune (+7.8) reaches opposition on Saturday, though it will be trickier to spot than usual that day as the Full Moon will only be a few degrees away. Spot the Ice Giant on other evenings of the week as it rises at around 18:55 BST. Close by is Saturn (+0.3), which rises at around 19:17 BST this week. It is increasingly well placed for observation as it moves towards its opposition next week.
- With the waxing Moon, we turn our attention to open clusters this week, starting with Messier 39 (+4.6) in Cygnus, first spotted by Aristotle around 325BC! He thought it to be a comet. Then we move to Messier 52 (+6.9) in Cassiopeia, before moving on to NGC 457, the Owl Cluster (+6.4) just beside the W asterism of Cassiopeia.
- The Moon is Full on Saturday – the Harvest Moon.
- The Sun currently has 1 active region and the sunspot number is 11.
- There are multiple visible evening ISS passes this week (click here for full ISS pass details).
Click here for the What’s Up! archive
Latest News
What's been going on at the Blackett Observatory
21st September
GCSE Observing: The eight pupils from the Hundred and Remove Astronomy GCSE classes joined GKWJ at the Observatory for the first observing session of the term. It was beautifully clear, but the Waxing Gibbous Moon was rather bright! It made a stunning first target in the 10-inch, with Sinus Iridum particularly interesting illuminated on the Terminator. The group went outside for a tour of the asterisms and pointers required for the course. Then back to the 10-inch for an unusually still view of Saturn with its moon Titan. The session ended with a view of the Andromeda Galaxy through binoculars. A great start to the season.
19th September
Friends Drinks: The new year kicked off as is tradition with the Drinks Party, this time celebrating 22 years of the Friends. A great turnout saw 50 Friends gather at the Observatory under cloudy, but not rainy skies! Food and drink was enjoyed by all, making a great start to the new academic year.
Open Day: GKWJ took the Wetton Meteorite Collection to the marquee in Court to entertain and inform the inquisitive parents and prep school pupils. The rocks were much enjoyed.
22nd June
Radcliffe Society: Traditionally, the summer meeting of the Radcliffe Society is our opportunity to launch rockets on the sports fields by the Observatory. However, ‘adverse atmospheric conditions and excess precipitation’ meant that Mission Control was forced to cancel all launches. Instead, 6 pupils came to the Dome and discussed all things Space and Astronomy with GKWJ, touching on the eclipse on August 12th, the trouble with near light speed space travel, and the dark universe. The group also enjoyed their final KitKats and hot chocolate of this academic year.
21st June
Friends Observing: 13 Friends joined GKWJ at the Dome for the Summer Solstice Celebration observing session. Hazy twilight meant that we started the session with a view of the First Quarter Moon in the 10-inch and the 127mm Maksutov that a Friend set up outside. We moved on to Venus, showing a clear gibbous phase. As the twilight slowly edged towards almost darkness and stars started to appear, we moved to the Double Double in Lyra, at 95x magnification, an obvious double, then at 140x and 220x, becoming an easy split Double Double. The theme of double stars continued with Albireo, then followed by a quick stop at M57, there, but very faint in the not dark sky, on to M13, a better sight, but lacking good contrast. Back to doubles, with Alpha Herculis, Rasalgethi, an easy split, and Zeta Herculis, a much tougher challenge. By this time some had departed, leaving the hardy few to keep going towards midnight, rewarded with a clearing and darker sky, we went to ‘the other globular’ in Hercules, M92, which was stunning, so much so, that we returned to M13, which now showed as the punchy ‘sprinkling of salt’ in the eyepiece. Then the challenge really started, with a Friend suggesting that we try for the Veil Nebula in Cygnus; in a first for the 10-inch, at least in recent times, we found the clear hazy veil structure of ‘The Witches Broom’, the western Veil Nebula, which really ‘popped’ with the OIII filter. Our final target was NGC 6826, the Blinking Planetary Nebula, a clear planet like ‘blob’ through the OIII, then with no filter, its name became apparent – stare straight at the object and you see the small, bright, central white dwarf star piercing through, but when you look to the side, the white dwarf vanishes and averted vision reveals the fainter, larger nebula, flick between the two views and the white dwarf blinks on and off. A fabulous, fun evening on the ‘worst night for astronomy’!!
All Sky Eye
The Latest View from the MCBO
The All Sky Camera operates from 30 minutes after sunset through the night until 30 minutes before sunrise. The latest image is updated automatically every 5 minutes and the time lapse video is refreshed each morning at around sunrise. Click the still image to view a large version
Latest Still Image
Most recent time lapse video
Sky Quality
The Latest SQM Chart from the MCBO
Thanks to generous funding from the North Wessex Downs National Landscape, the Blackett Observatory now has a fixed Sky Quality Meter (SQM) in place, measuring sky darkness every night throughout the year.
This is the latest chart of SQM readings, measured in magnitudes per square arcsecond (mpsas). The darkest reading to date is 21.1 mpsas, which equates to Bortle 4 and a naked eye limiting magnitude of +6.3. The sky will now be monitored constantly to see how the effect of light pollution is changing over time. A project to monitor sky quality across our local area is underway using NWD funded handheld SQM devices and volunteers from the Friends of the Marlborough Telescope.
The SQM chart updates every 15 minutes through the hours of darkness.
The yellow circles show the altitude of the Moon, if present.