Question: How do we know what time to eat by on Eight Precepts’ (八戒) days, in order to ‘not eat passing noon’ (过午不食)?
Answer: If you wish to be specific, for daily noon times, you can see sites like this: https://www.sunrisesunsettime.org/asia/singapore/singapore-february.htm. For example, for today, the 27th of February in 2023, which is the 8th day of the lunar month, as a Six Purification Day (六斋日) for observing the Eight Precepts, the table says the exact noon time is 1.18 pm. This is when the sun is exactly at the zenith, directly overhead, (which is thus not always at 12 pm). If you wish to abide by an estimated time, you can eat by a fixed time, perhaps between 11.30 am to 1 pm.
Note that for Mahāyāna (大乘) Eight Preceptors and Bodhisattva Preceptors (菩萨戒子), food and drink should be free from meat, alcohol, five pungent roots and animal ingredients, as detailed in the last Related Article. Bodhisattva Preceptors should consume as purely as possible every day, not just on Eight Precept days.
Question: How did those in the Buddha’s time know when it is noon?
Answer: They probably simply look to see if the sun is overhead yet. Note that in the Buddha’s time, there is only ‘one meal for one day’ (一日一餐) for the Buddha and the monastics, which is with alms taken near but before noon, as the Eight Precepts, among other Precepts, are observed every day. Nowadays, many lay Eight Preceptors eat breakfast too, as they live non-monastic lives, that might require more energy for work.
Eight Precepts Ceremony (For Precept Recipients)
Six Purification Days, Ten Purification Days And Three Purification Months
How Should All Aspiring For Buddhahood Eat And Drink?