The Astronomy of the Solar Pyramid

A visit to the Solar Pyramid will not only help to reconnect us with the rhythms of our natural environment, but stimulate us to consider to what extent the Sun is the ultimate source of the energy that powers our world.

Dr. Robin Catchpole, Institute of Astronomy, Cambridge

Sundials

Push a stick in the earth and the shadow of the stick cast by the Sun on the ground will move through the day, marking the passage of time. At midday, local solar time, the Sun will be due south and the shadow will lie due north of the stick (in the northern hemisphere). This will not make a good solar clock, because, during the year as we go from summer to winter, the Sun will be at a different height in the sky at the same Solar time and the stick shadow will lie in a slightly different direction.

This problem is overcome in the conventional sundial by aligning the pointer that casts the shadow, parallel to the Earth’s axis of rotation. This means that the pointer is aligned north south and is tilted at an angle identical to the latitude of the sundial site. At night, the pointer points very close to the Pole Star, at the place around which the night sky seems to rotate.

A sundial gives us time by the Sun, but this is no longer the sort of time we use to regulate our lives for the simple reason that “Sun hours” do not have the same length all the year round. This is the reason that a sundial has a table of corrections, referred to as the “Equation of Time”, which must be applied to the “Sun time” to get clock time. It is also the reason for the word “mean” in Greenwich Mean Time (GMT).

Adam, Richard and Dr Catchpole viewing the scale model of the Solar Pyramid at the Space Time Gallery of the Royal Observatory at Greenwich

The Solar Pyramid

The Solar Pyramid is both a work of art and a giant sundial with the southern arm pointing north-south and set parallel to the earth’s axis of rotation. The two secondary arms also have astronomical significance as one is aligned toward the point on the horizon where the Sun rises and the other to where it sets at the time of the Summer Solstice, which is the longest day of the year.

plan

The huge size of the Solar Pyramid allows us to draw a dial that can be read for every day of the year, although to avoid clutter, only the months of the year are marked. This means that it is possible to read Greenwich Mean Time directly from the dial as the shape of the hour and minute markers allows for the Equation of time. The markings also allow for the fact that the dial has a slope of 1:50 to allow rainwater to run off.

The dial also shows the time of sunrise and sunset at the Solar Pyramid and illustrates the difference in the number of hours of daylight from summer to winter.

When the dial is designed to show the time for each day of the year, it is no longer necessary to have the gnomon pointing parallel to the Earth’s axis of rotation. For example, the gnomon could be vertical or correspond exactly to the angle defined by a 3:4:5 right angle triangle, as does Kahfre’s pyramid at Giza, one of the the original inspirations for the project. We have decided to follow conventional sundial design, uniquely anchoring the angle of the Solar Pyramid to its latitude. The design of the dial is unique to both latitude and longitude.