peat, bog, witching hour, Ireland, Mayo

Watching the peat bog at twilight will be the event of each day when I return to Ireland next Friday. Here, I will be waiting for what photographers call the “sweet light”, which is literally a gift from the heavens. ©

Peat bog

A peat bog is a wetland that accumulates peat, a deposit of dead plant material—often mosses, and in a majority of cases, sphagnum moss.[1] It is one of the four main types of wetlands. Other names for bogs include mire, quagmire, and muskeg; alkaline mires are called fens. They are frequently covered in ericaceous shrubs rooted in the sphagnum moss and peat. The gradual accumulation of decayed plant material in a bog functions as a carbon sink.[2]

Bogs occur where the water at the ground surface is acidic and low in nutrients. In some cases, the water is derived entirely from precipitation, in which case they are termed ombrotrophic (rain-fed). Water flowing out of bogs has a characteristic brown colour, which comes from dissolved peat tannins. In general, the low fertility and cool climate results in relatively slow plant growth, but decay is even slower owing to the saturated soil. Hence peat accumulates. Large areas of landscape can be covered many metres deep in peat.[1][3]

Bogs have distinctive assemblages of plant and animal species, and are of high importance for biodiversity, particularly in landscapes that are otherwise settled and farmed.


Twilight is the illumination of the Earth’s lower atmosphere when the Sun itself is not directly visible because it is below the horizon. Twilight is produced by sunlightscattering in the upper atmosphere, illuminating the lower atmosphere so that the surface of the Earth is neither completely lit nor completely dark. The word “twilight” is also used to denote the periods of time when this illumination occurs.[1]

The further the Sun is below the horizon, the dimmer the twilight (other things such as atmospheric conditions being equal). When the Sun reaches 18 degrees below the horizon, the twilight’s brightness is nearly zero, and evening twilight becomes nighttime. When the Sun again reaches 18 degrees below the horizon, nighttimebecomes morning twilight. Owing to its distinctive quality, primarily the absence of shadows and the appearance of objects silhouetted against the bright sky, twilight has long been popular with photographers, who refer to it as ‘sweet light’, and painters, who refer to it as the blue hour, after the French expression: l’heure bleue.