SolarSystem Sun Card

 The Sun Card is the one physical thing a SolarSystem.city walk needs.

Its border is an AR target: point your phone at it and the app knows exactly how big the card is — which is what lets it draw each planet at true scale on the pavement in front of you. Mercury the size of a marble. Jupiter taller than you are.

It carries no city, no coordinates and no planet. Which planet you see is worked out from where you're standing and how far through a walk you are — so one card works on every walk we publish, in every city, indefinitely.

• CR80 wallet card, 85.6 × 54 mm, durable plastic

• No app to install — scan the code and the walk opens in your browser

• Works on every published walk, forever

• The walks are free. The card is the only thing you ever pay for

 Preview any walk first at solarsystem.city/explore