Wikipedia:Reference desk/Archives/Science/2023 December 7

= December 7 =

"I'm Julian and this is..."
I just came across this website. It explains what the Julian date is, but it also has a "Julian Date Calendar 2023", which is the day of the year mapped on to the date. As there are no weekdays we can't tell whether this is Julian or not, but in 2020 Tesco said this was Gregorian. There is a reference to "food service industries". Who is right? 92.29.73.147 (talk) 13:21, 7 December 2023 (UTC)
 * That weird website conflates Julian date and Julian calendar, which are not the same thing. The calendar that we usually use is the Gregorian calendar. The Julian calendar is similar to the Gregorian one in that is uses months and days within the month, just with different rules for leap years, which causes an increasing difference between the two. The Julian date is simply the number of days since 1st January, 4713 BC. What Tesco uses is simply the number of the day within the (Gregorian) year — I don't know whether people call this a "Julian date" (imho they shouldn't...). --Wrongfilter (talk) 13:55, 7 December 2023 (UTC)
 * See Julian day: "The term Julian date may also refer, outside of astronomy, to the day-of-year number (more properly, the ordinal date) in the Gregorian calendar, especially in computer programming, the military and the food industry". Martin of Sheffield (talk) 15:12, 7 December 2023 (UTC)
 * Yes, it's extremely common in the food industry for manufacturers to stamp the "julian code" on products, especially shelf-stable ones where there's no need for a best-before date. They avoid stamping the date itself as customers would balk at perfectly safe food that was "old", so it's very handy that so many vendors use the same methodology as people in the industry can readily read them. Matt Deres (talk) 19:00, 7 December 2023 (UTC)
 * A useful concept, but a terrible misnomer. -- Jack of Oz   [pleasantries]  21:35, 7 December 2023 (UTC)
 * I'm not sure I see why, unless your astronomy lab has some stock boys inadvertently using their julian date definition where they oughtn't. Matt Deres (talk) 15:54, 8 December 2023 (UTC)
 * It's based on the day of the year in the Gregorian calendar. Not the Julian or the Mayan or the Jewish or the Islamic or anything else. The only saving grace is that in 97 years out of 100 the day of the year is always the same in both Julian and Gregorian calendars, and in the other 3 years it's the same for the first 59 days. That's about a 97.79% correspondence, but it's wrong in principle. They've taken the term "Julian day" - which starts with 1 at a point where the 3 cycles of the Julian calendar coincided before historical times, and adds 1 for every subsequent day, continuing forever, not being reset to 1 each year - and applied a similar-sounding terminology to the Gregorian calendar, but without any regard to (a) the incommensurability between the calculation of Julian days and so-called Julian dates, which are reset every January 1, and never exceed 366, or (b) the complete inappropriateness of using the word "Julian" anywhere in the name. That's why it's a terrible misnomer. Also, approximately 97.79% of stock boys have never even heard of the Julian calendar; all they know is "the calendar" (= the Gregorian). -- Jack of Oz   [pleasantries]  17:02, 8 December 2023 (UTC)
 * No no no!  In 397 out of 400 years the length of the year is the same.   For the Revised Julian calendar (now there's a name to play with) the corresponding figure is 893 in 900.   For Gregorian v Revised Julian the figure is 3587 in 3600. 2A00:23C7:9CD1:3901:7762:1B1D:468A:BF09 (talk) 17:46, 8 December 2023 (UTC)
 * You seem to have misread my post. I said "the day of the year", not "the length of the year". -- Jack of Oz   [pleasantries]  08:07, 9 December 2023 (UTC)
 * I see your point, though grocers at least do not use their system as a calendar. Matt Deres (talk) 22:40, 9 December 2023 (UTC)
 * Implicitly "hello! I'm Julian and this is my friend Sandy!" . . A dated catchphrase. . dave souza, talk 22:56, 9 December 2023 (UTC)

Atmospheric pressure changes and water boiling
Do changes in atmospheric pressure, as reported in weather forecasts, affect the boiling point of water at ground level (e.g. increasing or decreasing it, ultimately affecting the boiling time in a kettle)? 212.180.235.46 (talk) 14:23, 7 December 2023 (UTC)


 * Any changes in atmospheric pressure (measurable at ground level with, say, a barometer) would affect the boiling point of water according to its phase diagram (refer to Water (data page) for specifics). However, even at the absolute extremes in atmospheric pressure differences, the boiling point would only decrease by 4.3 °C (39.7 °F) 4.3 °C (7.7 °F) (at 25.69 inHg) or increase by 1.9 °C (35.4 °F) 1.9 °C (3.4 °F) (at 32.03 inHg). I don't think you would end up saving more than 30 seconds compared to normal atmospheric pressure. Reconrabbit (talk) 14:46, 7 December 2023 (UTC)
 * You seem to have made an error in your unit conversions. A difference of 4.3 °C is equal to a difference of 7.7 °F, not 39.7 °F, and a difference of 1.9 °C is a difference of 3.4 °F, not 35.4 °F. You don't add 32 when you're converting a temperature difference, as opposed to an absolute temperature. CodeTalker (talk) 19:14, 7 December 2023 (UTC)
 * My bad, I was using the automatic conversions, which assume absolute units. Reconrabbit (talk) 19:24, 7 December 2023 (UTC)
 * If you're using the Convert template, you can use the unit "C-change" rather than "C" to convert a temperature difference. CodeTalker (talk) 19:30, 7 December 2023 (UTC)
 * At the top of Mount Everest, the boiling point of water is about 68°C (154°F) due to the decrease in atmospheric pressure with height above sea level. Philvoids (talk) 19:41, 7 December 2023 (UTC)