In a modern country like Singapore, people still rely on cultural traditions to determine the dates to celebrate different festivals. The Chinese use the Chinese calendar to determine the dates for Chinese New Year and Mid-Autumn Festival, a new moon to mark the beginning of a new year and full moon for mid-autumn. The Muslims use a pure lunar calendar to determine Islamic festivals like Ramadan, which starts on the first sighting of a crescent moon. Meanwhile, Deepavali is determined from the day of the new moon on the Hindu lunisolar calendar. Lunar calendars exist in many versions, and while some are synchronised with the solar calendar every now and then, there will still be differences in the number of days in a year. As such, different cultures in Singapore would celebrate their significant festivals at different dates every year. Nonetheless, we still depend on the phases of the moon to tell us the date.

One of the upcoming festivals for Singapore Chinese is the Mid-Autumn Festival. The full moon that will occur on 1 October 2020 also happens to be dubbed as Harvest Moon in western cultures. While the name “Mid-Autumn” explains that this is the full moon that falls in the middle of autumn (September to December), Harvest Moon is so named because of how it helps farmers. In other times of the year, the moon rises from the Eastern horizon 50 minutes later each day. That is, if the moon rises at 7pm today, it will only rise at 7.50pm tomorrow. But the first full moon after the vernal equinox would rise just 30 minutes later than the previous day. This early rising would mean that farmers could continue working even after the Sun has set as the sky is almost continuously bright. However, this phenomenon can only be observed in temperate countries and not observable near the equator, where Singapore is. Thus, on the day of Mid-Autumn, our full moon would only rise 50 minutes later than the day before, like on any other day.

Moving forward on the calendar we notice a second full moon on 31 October. This is commonly known as a Blue Moon! While the moon does not really turn blue, it might reflect people’s moods as the month feels longer than usual, having experienced 2 full moons in in the same month. The moon, however, may appear blue only when dust particles of a certain size are able to scatter red light away and allow blue light to reach our eyes. This sort of particles may be made up of volcanic ashes. But who would be in the mood to observe the moon in the middle of a volcanic eruption?

Speaking of Blue Moon, we recall a Super Blue Blood Moon in on 31 January 2018. This is a coincidence of 3 phenomena on 1 date. While this coincidence might just happen again on 31 January 2037, Super Moons, Blue Moons and Blood Moons occur in different frequencies. A Super Moon refers to the full moon that is the closest to the Earth in a given year. This happens every 13 months or so. A Blue Moon occurs once in about 3 years, similar to that of a Blood Moon. But while the occurrence of a Blue Moon is due to coincidences in lunar and solar calendars, the occurrence of a Blood Moon is due to the oscillation of the plane of the Moon’s orbit.

A Blood Moon refers to a total lunar eclipse. For a lunar eclipse to happen, the Moon, Earth and Sun must be in a straight line in 2 dimensions. When the three bodies are aligned, the smaller body – the Moon hides in Earth’s shadow. This shadow, however, isn’t black, it is red! Do you recall seeing red sunsets as the sunlight passes through the thick atmosphere, especially when it is cloudy? This red light goes beyond the Earth and is present in its innermost shadow. Thus, when the Moon enters this innermost shadow, it has red light shining on it. The colour red reminded folks of blood baths that happened during war, hence the name Blood Moon.

There is much more to be observed regarding the moon. If you ever wondered what the significance of the horoscopes is, try to take note of the stars that the full moon rises close to each month. There are some applications on your PC or phones that can help you to identify stars and constellations, and to calculate when will the moon be visible in the night sky. I recommend PC users to try using Stellarium (free), and SkySafari for those who wish to stargaze with your phones. Do note that SkySafari is free of charge on Google Play Store but comes at a charge on iOS App Store.

Did we get you interest in observing the night sky? Follow Science Centre’s Observatory team on www.facebook.com/SCObservatory and join us on our Friday evening stargazing sessions! The Observatory is open every Friday, 7.45pm to 10pm, except on public holidays.

Written by Mok Li Hui
Illustrations by Toh Bee Suan