As the full moon rises tomorrow (Friday March 14), it will be a special sight for those in Aotearoa New Zealand. It will also be worth a look for people along the east coast of Australia.
Author
- Tanya Hill
Honorary Fellow at University of Melbourne and Senior Curator (Astronomy), Museums Victoria Research Institute
Rather than being full and bright, the Moon will be partway through a lunar eclipse, the first of two lunar eclipses to occur this year.
New Zealand is in for a treat as the Moon will rise during totality - when the Moon passes completely into Earth's shadow. Instead of turning dark, the Moon takes on a reddish glow that's colloquially referred to as a "blood moon".
Along the east coast of Australia, totality will happen while the Moon is still below the horizon; by the time the Moon rises, it will be in part-shadow.
A red Moon in Earth's shadow
When it's a full moon, the Sun and the Moon are located on opposite sides of the sky. With Earth in the middle, it can cast a large shadow blocking the Sun's light from reaching the Moon.
However, during most full moons we don't see an eclipse because the Moon's orbit is slightly tilted - by just five degrees - compared to Earth's orbit around the Sun. Most months the full moon passes either above or below Earth's shadow. But twice a year, the path of the Moon takes it through the shadow instead.
When the Moon is fully immersed in shadow, the reason it turns red is entirely due to Earth's atmosphere.
The first eclipse from the Moon
The Blue Ghost Mission 1 , which successfully landed on the Moon on March 2, will be the first to image an eclipse from the Moon. As we experience the lunar eclipse, the Blue Ghost 1 lander will see a total eclipse of the Sun thanks to Earth moving in front of it.
Being on the Moon during totality and looking up at the Earth, it should see the atmosphere lit up as a ring of red.
Only the low-wavelength red sunlight passes through the atmosphere because the bluer light is scattered away. This is also the reason why sunsets have red, orange and pink hues.
Importantly, the atmosphere also refracts or bends the light, redirecting it into Earth's shadow and making the Moon appear red.
When and where to look
Lunar eclipses are brilliant to watch - they are perfectly safe and you don't need special equipment. Since the Moon will be low in the sky, you will need a clear view of the eastern horizon, perhaps from somewhere high up. It's a leisurely event, so it's also great to have good company.
Since this eclipse happens at moonrise, you can use the website timeanddate.com to check the moonrise time for your location and also to determine the eclipse magnitude, which is a measure of how much of the Moon is in shadow.
An eclipse magnitude of 1 or more means the Moon is fully in shadow or has reached totality.
If it is less than 1, it refers to the greatest fraction of the Moon's diameter that is eclipsed. Imagine a diameter line across the Moon: where the edge of the shadow falls along that line will denote the magnitude of the eclipse.
Across New Zealand, the Moon will rise during totality. The farther north, the longer totality will be. By the time the Moon moves out of the shadow, twilight will have ended and the sky will be lovely and dark for the later part of the eclipse.
On the east coast of Australia, the eclipse will be visible against the bright twilight sky. This will make it much harder to see from southern New South Wales, Victoria and Tasmania, since only a small part of the Moon will be in shadow.
Trick of the eye
But wait, there's more. Watching the Moon when it's low on the horizon also creates an interesting effect called the Moon illusion.
Our brains trick us into thinking the Moon is much bigger than it usually is. But if you use your thumb to cover up the Moon when it's low in the sky and then measure it again later in the evening when the Moon is up higher, you'll see the Moon hasn't really changed in size at all.
The illusion likely occurs because we instinctively think the sky is shaped like a dome and that the Moon is closer to us when it's overhead and farther away when it's near the horizon. After all, that's what happens when a bird flies off into the distance.
But the Moon is much farther away than a bird; its distance doesn't change over the course of a night.
If our brains are telling us the Moon is farther away when it's on the horizon, the Ponzo illusion demonstrates why we are tricked into thinking it appears bigger. In the image below the two moons are exactly the same size, but the perspective provided by the railway tracks makes us see the horizon one as larger.
If you aren't able to see this eclipse, the second total lunar eclipse for 2025 will happen during the early hours of September 8.
It will be visible from across Australia, while New Zealand will see the eclipsed Moon setting at sunrise: almost an exact opposite to tomorrow's eclipse.
Tanya Hill does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
