Cassini’s Meridian Line in Bologna !


Cassini’s Meridian Line, anybody ? If you’re not excited by Astro History Opportunities you can skip this. But here we are in the Basilica di San Petronio. And there, at our feet, (pink arrowed in this pic) is the Meridian Line set there by master astronomer Gian Domenico Cassini in 1655. It’s orientated exactly North-South, as Meridians have to be. And damned difficult that was to achieve in a Church which was aligned completely differently. The line just skirts the huge columns which support this giant place of worship.

And up In the roof (also arrowed) is the hole through which the Sun will send a shaft of light on to the Meridian line at EXACTLY local astronomical mid-day.

Basilica di San Petronio

So … in the Basilica … looking up, here’s a close -up of the small hole which acts as a kind of camera lens to project the Sun’s image on to the Church floor.

Hole in ceiling

So … in Cassini’s basilica … here we are, having planned this moment with military precision, watching a number of people milling about, wondering where to look. Yes, the image of the Sun is supposed to hit this line on the floor at midday – but WHERE on the line ? Nobody seems to have a clue. I figure it’s almost midwinter, so the Sun is low in the sky, so its image has to fall way back on the line, far from the entrance hole. So we take up a guessed position, but so far, at 11.50am, there is no sign of the Sun. Maybe it’s disappeared back into the morning mist. I feel like dear old Sir Patrick Moore, waiting and hoping for a fabulous cosmic event which may be wiped out by a couple of terrestrial clouds. It’s 11.54 am. Local astro midday is at 11.58. Can a miracle happen in the next 4 minutes ? If you’re viewing this stereo you’ll see the people in it have moved around between my two -Phone shots, left and right. So they flash weirdly. Apologies if this hurts your brain. I like it because it reminds me how ephemeral we humans are. Cassini’s Meridian is kinda eternal.

Basilica - internal view - stereo

So … back in the basilica …. it’s 11.47 am and suddenly – as if by magic – there it is ! Cassini’s giant pinhole camera delivers a perfect image of the Sun on the marble floor. The Sun is moving from left to right, out there, so its image is moving from right to left down here, fast approaching the centre line.  It’s beautiful !!! Cassini was so clever – there is only JUST enough space for the essential ray of light to get through the columns – and only for a minute or two. You can see the shadow of in of the columns on the right hand edge of the Sun’s image here. If you come to Bologna, folks, you have to see this ! Unforgettable ! And Cassini was able to confirm some good science from carefully measuring using this giant sundial. In those days there was significant doubt whether Earth revolved around the Sun, or the other way round. Cassini helped to nail it.  Notable that although we DID plan the visit carefully, it was purely by the grace of God that we saw anything, because one minute after this picture the Sun faded out of sight behind clouds. Grateful !!!

Cheers Astro pals.

People view sun's image

The image of the Sun. Captured at 11.57 am today in beautiful Bologna. As provided by Il Dottore Cassini in 1655.

Sun's image on Basilica floor