September 08, 2022

More Purple Sunset Moons in Autumn

Why the Moon looks prettier more often as summer begins to leave us

Last night, we were treated to one of my favourite views of the sky: as the Sun sets and the sky is still light out, the Moon appears low on the horizon in front of a purple sky.

I don’t know if there’s a real name for this, but let’s call it a “Purple Sunset Moon”.

And what I’ve found is that we get more of these views in Autumn than other times of year. Let me show you…

Moon at Sunset in Autumn

Moonrise Time

The Moon rises later each night. For example, if you go outside the night of the Full Moon and watch the Moonrise at 6:00 PM, it will rise later the next evening.

By how much? A rule of thumb I use is about 45 minutes

So, if the Moon rose at 6:00 PM on Monday, it will rise at:

  • 6:45 PM on Tuesday
  • 7:30 PM on Wednesday
  • 8:15 PM on Thursday

And so on.

This is just a rough estimate though. It’s not exact. And what I’ve found is that it varies throughout the year.

Moonrise in Spring

Moonrise in Spring

Around May is when I start doing Moonrise workshops. I try to choose days that are right around sunset time, and also aren’t too early or too late. 

Below are the Moonrise times around the Full Moon in May 2022. In parentheses is the amount of time the Moon rises later from the night before:

  • May 14: 6:38 PM
  • May 15: 8:01 PM (1h 23m)
  • May 16: 9:25 PM (1h 24m)

My rule of thumb of 45 minutes here is almost broken! The Moon rises nearly double that time in May.

What does this mean? Well, on May 14, the Moon would rise well before sunset and we wouldn’t get that beautiful Purple Sunset Moon. The next night, May 15, the Moon rises at sunset time, and we would get it. But the next night, the Moon would rise when it’s quite dark.

So in May, we get one Purple Sunset Moon. 

Moonrise in Autumn

Let’s compare this with September Moonrise times:

  • September 7: 6:27 PM 
  • September 8: 6:59 PM (32m)
  • September 9: 7:25 PM (26m)
  • September 10: 7:47 PM (22m)
  • September 11: 8:07 PM (20m)

Look at the drastic difference in Moonrise times compared to May! Rather than the Moon rising almost 1.5 hours later, it’s only around 25 minutes.

What does this mean? If the Sun sets around 7:15 PM this time of year, we have multiple days in a row where we bask in a Golden Hour sky and the nearly Full Moon will appear low on the horizon.

More Purple Sunset Moons in Autumn. 

Moonrise at Mordecai Richler Gazebo

Why Such a Drastic Difference?

Not ashamed to say this: I don’t know! 

That doesn’t mean nobody knows – there’s certainly an answer out there. And if you know what it is, let me know!

But this is why I love studying this stuff as an amateur: you stumble upon these things you notice, and try to find an answer for them. Rather than the opposite: learning about the answer in a textbook and then noticing it.

So, as the summer winds down and Autumn begins, notice the sunset and Purple Sunset Moons.