For anyone who also reads my photo blog, you might have seen that I went out around sunset last night to see if there were any interesting photos to be taken.

Before I left the house, I checked the official time of sunset on the BBC Weather website, and found it to be 9:04pm. I wasn’t really sure how “sunset” is defined, so I left the house early to cover myself.

My observations on the evening didn’t really help me deduce what is meant by “sunset” as it’s hard to tell when the sun goes below the horizon when there’s a gorge, some cliffs and a tall forest in the vicinity. I also wasn’t sure if it was the time that the leading edge, trailing edge, or midpoint of the sun touched the horizon. So I looked it up on Wikipedia.

In astronomy the time of sunset is defined as the moment the trailing edge of the sun’s disk disappears below the horizon in the west. Due to refraction of light in the atmosphere, the ray path of the setting sun is highly distorted near the horizon making the apparent astronomical sunset occur when the sun’s disk is already about one diameter below the horizon. Sunset should not be confused with dusk, which is the moment at which darkness falls, when the sun is about eighteen degrees below the horizon. The period between the astronomical sunset and dusk is called twilight.

So now you know. The official time is not only defined in a vague way (what’s the horizon?) but also hard to measure.

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