Public Open Evening at the IoA

Student volunteer at the Institute of Astronomy's weekly Public Open Evenings, University of Cambridge

The Institute of Astronomy opens its doors to the public every Wednesday evening from 7 pm to 9 pm, between October and March. I am one of the student volunteers who help host these weekly Public Open Evenings, welcoming members of the public for an evening of observing under the Cambridge sky.

I operate the Institute’s 12.5-inch Dobsonian telescope to show visitors Jupiter and its four largest moons — the Galilean satellites Io, Europa, Ganymede, and Callisto — first observed by Galileo Galilei in 1610. Unlike the 16-inch telescope housed in the main dome, this Dobsonian mount has no motorised tracking to compensate for the rotation of the Earth, so I manually adjust its right ascension roughly once a minute to keep Jupiter centred in the eyepiece as the planet drifts across the sky. The hands-on nature of the instrument makes for a memorable experience: visitors not only see the moons of another world for themselves, but also gain a tangible sense of the Earth’s rotation in real time.

The 12.5-inch Dobsonian telescope on the grounds of the Institute of Astronomy, University of Cambridge.

Alongside the telescope, I use a hand-held green laser pointer to guide visitors around the night sky, tracing out constellations and pointing to individual stars and planets while describing what they are seeing. Green laser pointers are the tool of choice for stargazing because, at equal power, green light appears far brighter to the human eye at night than light of most other colours — the human eye is most sensitive to wavelengths near the green part of the visible spectrum.

Using a green laser pointer to indicate a star while describing the night sky to visitors at the Institute of Astronomy.

Throughout the IoA site, ambient illumination is provided exclusively in red. The rod cells of the human retina — responsible for vision in low light — are far less sensitive to red wavelengths than to the shorter wavelengths of blue and green light. Red lighting therefore allows the eye to remain dark-adapted, preserving its ability to perceive faint sources of light in the night sky, such as dim stars and the subtle details visible through a telescope eyepiece.

The historic Northumberland telescope at the Institute of Astronomy, University of Cambridge, photographed under the red ambient lighting used throughout the site to preserve dark-adapted night vision.