Some like to celebrate the summer solstice sunset, some the sunrise, and others what is called the moment of maximum tilt. At 3.57 pm today, on the summer solstice 2023, this will be the moment at which the earth will be at its maximum tilt towards the sun in the northern hemisphere.

The earth's tilt, or obliquity as it's sometimes caused, away and towards the sun, is what causes our seasons. This means, at its most extreme, at the North Pole, at the summer solstice, the sun does not set and the South Pole, at the same time, sees no sunrise. 

The tilt at which the planet spins is also the subject of much debate amongst scientists and astronomers some of whom believe that without it, Earth would not have become home to life. 

Why does the tilt make a difference?

It's not just that without it we would have no seasons, but life itself might not be possible. Imagine, for instance, an upright Earth, without the slouch. An article in the New York Times, published last summer, outlined the theories of Dr René Heller, an astronomer at the Max Planck Institute for Solar System Research in Germany.

"The hemispheres," it said, "would never dip toward or away from their star. Instead, the poles (which always point toward the frigid depths of space) would be so cold that carbon dioxide would be pulled from the sky, an effect, Dr. Heller argues, that would cause the planet to lose its precious greenhouse gas so that liquid water could never form."

According to a study by Purdue University, a modest axial tilt, which is what Earth has, helps increase the production of oxygen, which is vital for life.  Report author Stephanie Olson said: "The bottom line is that worlds that are modestly tilted on their axes may be more likely to evolve complex life. This helps us narrow the search for complete, perhaps even intelligent life in the universe."

What is the exact angle of tilt of the Earth?

Earth currently has a 23.4° axial tilt. Most scientists say that the exact angle doesn't matter - it's simply having a modest tilt or obliquity, neither too small nor too large, that makes the conditions for life more likely. 

READ MORE: 7 Spots in Scotland to witness the solstice sunset

READ MORE:  Simmer Dim: the magic of midsummer twilight on Unst

Does Earth's tilt change at all?

Scientists recently discovered that it has changed in recent years - and that we humans are responsible. The tilt shifted by 80 centimetres between 1993 and 2010 because of the amount of groundwater we pumped from the planet's interior. Altogether we removed 2,150 gigatons of water from natural reservoirs in the planet's crust. 

But this is minor compared with the larger shifts that have occurred in our planet's history, during which the angle of tilt has varied between 22.1 and 24.5 degrees.

Research published in the journal Geophysical Research Letters in 2021 also showed that glacial melting associated with the climate crisis was causing such a shift in the poles.

Earth's tilt is currently around halfway between its historical extremes and slowly decreasing. It was at its maximum around 10,700 years ago and will rich its minimum 9,800 years from now. 

Does the change in tilt impact the climate?

Yes, as the angle decreases seasons tend to become milder, with warmer winters and cooler summers, until, eventually snow and ice build up and the Earth cools further.

Why isn't the summer solstice the warmest day of the year?

Because of what is known as "seasonal lag". It takes some time for the planet's land, and in particular its seas, to warm up, so temperatures lag behind the sun hours experienced.