HPR4178: Today I learnt (2024-07-27)

TIL 1: Movement in the galaxy and mass extinctions Throughout the history of this planet there have been a number of mass extinctions. There are various theories about what has caused this. I came across one of these recently. The solar system doesn't orbit the galaxy in parallel with the orientation of the galactic plane. There is a difference in the two orientations in other words. Sometimes it's in the galactic plane, and at times it's "above" or "below". When in the plane the solar system is more protected from gamma ray bursts originating in the galaxy than when it's not. The period of rotation is the Galactic year and is about 225 million years long. The thinking is that when less protected perhaps the cosmic radiation has effects on life on Earth. There seems to be a correlation between these events - gamma ray exposure and extinctions - though there is no certainty that this is so. Extinctions may have multiple causes, so the relationship cannot be simple. I have provided a few links that might give useful leads if you want to find out more. TIL 1: Links Wikipedia: Extinction event Wikipedia: Galactic plane Wikipedia: Galactic year Physics Stackexchange: What angle does our Solar System make with The Milky Way? Wikipedia: Shiva hypothesis International Journal of Astrobiology: The galactic cycle of extinction TIL 2: Which is correct - ml or mL? What is the correct abbreviation for millilitre? I watch a YouTube channel about cooking, from Canada. It is Glen and Friends Cooking The show ends with a recipe, and in some cases the measurements are metric. Often it's something like 540 mL can of tomatoes. Having done Chemistry at school and university we were taught the abbreviated form of millilitre is ml, but today I learnt that mixed case mL is also permitted and is preferred in the USA, Canada and Australia, while Europe prefers lower case ml. It's a similar situation with other fractions and multiples of a Litre such as dl and dL for decilitre and even for quettalitre (1030 litres) - Ql and QL. TIL 2: Links Glen and Friends Cooking Wikipedia: SI prefixes applied to the litre TIL 3: Meibomium glands These glands are a type of sebaceous gland in the eyelid. Sebaceous glands in general produce secretions to protect the skin and hair. On the skin they are part of the hair follicle and produce an oily substance called sebum (from the Latin word for tallow used to make candles). The Meibomium glands are found along the edge of the eyelid adjacent to the eyelashes. The oily substance they produce helps to prevent the eye surface from drying out by mixing with the tears. It also helps the eyelids to make a