We know that some religions use lunar calendar to celebrate special events, like what we see when Muslims around the world determine the Eid al-Fitr day that falls on the first day of the month of Shawwal based on Hijri Calendar (a lunar based calendar), or the breathtaking picture of the full moon above the Borobudur Temple that we see in Vesak Day, the day when the Buddhists celebrate the birth, enlightenment and passing away of the Lord Gautama Buddha.
Fig. A: Full Moon above The Borobudur Temple, picture taken from www.asean-china-center.org
Not before the time when i started to see more the night sky, i realized that actually Christians use lunar calendar too, particularly when we celebrate Easter. You will see full moon in the week before Easter Day that always falls on Sunday.
It confused me to notice this as I knew the fact that Christians use either Gregorian or Julian Calendar (those two calendars have also another interesting stories) instead of using lunar calendar to celebrate special events. But the full moon that we always see in the week before Easter day proves that the method to determine the date of Easter must based on the moon’s phase, or, at least, the moon phase is one out of many variables that are used in the equations.
"The date of Easter Sunday is usually the first Sunday after the first Full Moon occurring on or after the March equinox."
That is the formula we can use to determine the Easter day. The March Equinox (astronomically) usually falls on March 19, 20, or 21. However, some church put a consensus that the March Equinox is March 21, ignoring the astronomical calendar (and some people are still having big debate on the method being used, maybe i will write a blog post about it). Thus the calculation of easter date is actually a combination between Lunar and Gregorian calendar. You can see the table below telling us the Easter day vs Full Moon occurrence in the last 10 years.
Fig. B: 2008 - 2018 Full Moon after March Equinox and Easter
So if you are a Milky Way photo hunter, you know it’s not good to have excessive light of the full moon in your frame, since you will need to take long-exposure shot. Unless, the Moon and the Milky Way is not in the same frame.
There is one cryptic word we find in the formula, and that is equinox. To understand what equinox is, we need to understand about how the Earth travels around the Sun.
One fact we need to know is that our planet's axis of rotation is tilted at an angle of 23.5 degrees relative to our orbital plane – the plane of Earth's orbit around the sun.
Fig. C: Eart's Tilt. Copyright: timeanddate.com
This caused the duration of day and night is not the same every day in either northern or southern hemisphere. You can see the illustration below:
Fig. D: Equinox and Solstice. Copyright: timeanddate.com
You can see the image above that if we are living in northern hemisphere, we will have:
1. The longest day of the year in June (June Solstice)
2. The same duration of day and night in September before Autumn (Autumn Equinox)
3. The longest night of the year in December
4. The same duration of day and night in March before Spring (Vernal /Spring Equinox)
Here is one explanation why you will have the longest night of the year in December (see picture below). I enlighten the part of the earth (the bright one) that receives the light of the sun in December. Let's say that you are living at point L. As the earth rotates, you will move along the green-yellow-dotted line. The green line refers to the duration of the day time, and the yellow line refers to the duration of the night time. You can compare the length of the green and the yellow line, and here we see that the green-dotted line is approximately 3 times longer than the yellow one. That means you are having a longer night time than the day time.
Fig. E: The longest night in December