Summer Solstice is on 21 June at 03:42 BST in 2025 and the Sun will appear highest in our sky for the year when due south. It will appear overhead at the Tropic of Cancer at a latitude 23.4°N due to the Earth’s axial tilt of the same amount. This is the start of “astronomical summer”.
We tend to think of these things as fixed but in fact the dates, times and angles all change. Why?
The date and time of the Summer Solstice varies between 20 – 22 June because our calendar doesn’t match exactly the Earth’s orbit around the Sun – by about a quarter of a day a year – and that’s why we have leap years, to sort the discrepancies out. On a leap year the Solstice will tend to fall on 20 June.
The Earth’s axial tilt in relation to its orbital plane also changes over time. It is currently around 23.4° but varies between about 22.1° and 24.5° over a 41,000 year cycle. The Tropic of Cancer is actually drifting slowly south at the moment at a rate of about 15m per year.
The Earth is further from the Sun in northern hemisphere summer than in winter, but that doesn’t necessarily mean southern summers are warmer. The difference in distance isn’t large enough to have a major impact and various other factors determine the temperature, such as land mass distribution and the effect of oceans.