THE last time Britons could see Venus cross the Sun was in 1283.
But no-one knew to look out for it then.
This time, local enthusiasts have been waiting excitedly for months, even years.
And they're inviting anyone who shares their interest to join them.
Huddersfield Astronomical and Philosophical Society is hosting an open morning at its observatory off Blackmoorfoot Road, Crosland Hill, next Tuesday.
The Venus Transit is being called the biggest event in the sky since the total solar eclipse in 1999.
Society president Lisa Jeffries says: "We'll have a battery of telescopes with solar filters attached, as well as our new solar telescope.
"Weather permitting, we'll start at 6am and finish at midday."
At 6.29am, shortly after sunrise, the planet will appear as a small black disc, silhouetted against the sun as it makes its first "transit" since December 6, 1882.
It will take six hours for the Earth's nearest planetary neighbour to cut a slow, left-to-right diagonal path across the southern part of the Sun, before disappearing at 12.24pm.
Entry to the observatory is the usual price of £2 ( £1 concessions).
"Given good weather, the entire phenomenon will be visible from Britain - the first time since 1283, when people didn't know to watch out for it," says Lisa, 39, a chemist at Huddersfield University who buys scientific equipment.
If it's cloudy, nothing at all will be seen - and club members will try to find internet sites with webcasts, as seen from elsewhere, so guests can watch the action on screen.
BBC TV and The Open University crews are set to be at the observatory.
The next transit isn't until June 6, 2012, but it won't be seen by UK skywatchers. They will have to wait until 2247.
Visitors to the Crosland Moor observatory are invited to simply turn up.
* Venus is the name of the Roman goddess of love and beauty.
* It is known as the Morning Star as well as the Evening Star.
* Orbit: 108m kilometres from the Sun, about two-thirds of the Earth's orbit
* Diameter: 12,100 kilometres, about 95% the size of Earth.
* Mass: About 80% that of Earth.
* Length of year: 225 Earth days
* Surface: Craters and volcanoes
* Atmosphere: Mainly carbon dioxide, with some nitrogen and virtually no water vapour, layers of sulphuric acid cloud many kilometres thick
* Atmospheric pressure: 92 times that of Earth
* Surface temperature: 475°C.