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Blackrock Castle Observatory

The 19th century Blackrock Castle is situated in Cork City, on the banks of the River Lee, about two kilometres from the City centre. It now houses an observatory, a visitor centre and a restaurant.

In the courtyard, inside the main entrance gate to the castle, are two modern dials, a 'Horizontal' or 'Garden' sundial set on a wooden pillar and an 'Analemmatic' or 'Human Interactive' sundial laid on the ground.

The Horizontal Dial
The sun reaches its highest point above the horizon at noon at all places along the same North/South line of longitude. It follows that at that instant, in places to the east of the dial, noon has already passed, and in places to the west, noon has not yet arrived. There is one hour of time difference for every 15 degrees of longitude difference. Blackrock Observatory is 8 degrees 24 minutes of longitude west of the Greenwich London Observatory, which is equal to 34 minutes of time. When it is 12:00 noon sun time at Greenwich it is 11:26 A.M. sun time at Blackrock. There is a 'noon gap' in the sundials time ring to compensate for the gnomon (the triangular shadow caster) thickness. When the shadow of the gnomon fills the 'noon gap' it is 12:00 noon sun time at Blackrock, but the time ring has been rotated to show 12:34pm, this is the sun time at Greenwich.

The noon gap has been labelled 'GMT' which stands for 'Greenwich Mean Time' which we use for our clocks during the winter. This dial does not show GMT. In order to derive GMT from sun time requires a 2-stage calculation. The first stage is the addition of the longitude correction element - this has been 'built-in' to the dial by the rotation of the time ring. The second stage is the addition/subtraction of the 'Equation of Time' element - shown on the graph on the dial - which varies on a daily basis between 16 minutes to be added in February and 14 minutes to be subtracted in November.
In summer when our clocks are set one hour 'on' we have to add this hour as a third element to get clock time - Irish Summer Time. For more information on the Equation of Time see my webpage at

The exhortation "Whatever you do, do it with the zeal of a warrior and the sensitivity of an artist" is believed to be part of saying attributed to Shri Mataji Nirmala Devi the founder of Sahaja Yoga Meditation.

The Analemmatic dial

Sundials come in a wide range of different types with the 'Garden' variety being the most common, but the most popular for modern parks and public areas is the 'Human Interactive' sundial which uses a person's own shadow to show the time. The user stands on the date scale on the appropriate date and their shadow points to the time. In the case of this dial, in order to show an approximation to Irish Summer Time the time ring has been rotated and shows 1:34 p.m. when it's 12:00 noon suntime at Blackrock.

For a full description of the difference between suntime and clock time click on this link

Lat 51° 54' North  Long 8° 24' West

Irish Grid   W   1 72470    072040

If you know the location of a sundial in Ireland (NOT a mass produced DIY Store garden ornament) please email it to me (Click here to email M.J.Harley) - a member of British Sundial Society
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