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What are equinoxes and solstices — and why do they matter for the light in your home?

Four dates a year mark the sun's extremes and turning points. Know them, and you can compare how light falls in your home at the year's two opposite poles, instead of guessing from a single sunny afternoon.

Why four dates are enough to understand a whole year

The sun's path shifts gradually every day of the year, but it moves between two extremes and passes through the midpoint twice. The four key dates capture both extremes and both midpoints:

• Summer solstice (around 21 June in the northern hemisphere). The sun reaches its highest point in the sky and is up the longest. The year's brightest day.
• Winter solstice (around 21 December). The sun sits at its lowest and is up the shortest time. The year's darkest day, and the one where low sun reaches deepest into south-facing windows.
• Spring equinox (around 20 March) and autumn equinox (around 22 September). The sun's path here sits midway between the two extremes — a kind of "average day" for sun height, even though the actual amount of light is still shaped in practice by the season's weather.

If you know how the light falls in your home on these four days, you've effectively covered the whole range the house will experience over a year. Everything in between sits on a sliding scale.

How low does the sun sit on 21 December?

It depends on latitude, but the pattern is dramatic the further you are from the equator: at around 55-56° N — roughly Denmark's latitude — the sun's highest point in the sky that day is typically only 10-15 degrees above the horizon around midday. By comparison, at the same latitude the sun at the summer solstice can stand 55-60 degrees above the horizon. That difference — between a flat, grazing angle and a high, near-vertical one — is the whole reason a window can feel sun-drenched in summer and shadowy in winter, even though it faces exactly the same way all year.

The low December sun also has an advantage: it can reach far into a south-facing room where an ordinary roof overhang or balcony wouldn't shade it at all, because the sun comes in at such a flat angle. That's often deliberate in older houses — an overhang sized to shade the high summer sun, while letting the low winter sun pass freely in and warm the room.

What is an equinox?

An equinox is one of the two days a year when the Earth's axis tilts neither towards nor away from the sun, seen from the middle of the day. The result is that day and night are roughly equal length wherever on Earth you are — hence the name. The sun's path that day is a good "neutral" reference: neither the extreme summer nor winter situation, but the midpoint.

For a daylight study of a home, the equinox is useful because it gives a comparable baseline without either seasonal extreme. If you're comparing two homes, or two versions of the same renovation, the equinox is often the fairest point to compare on — it's neither the best nor the worst case for either of them.

Will I get sun in the garden in winter?

That's the question the winter solstice answers best, because it's the year's hardest test. If a terrace or garden gets direct sun around 21 December, it will get it for essentially the rest of the year too — the sun sits lower on that date than on any other. Conversely: if something shades the garden in December — a neighbouring house, a tall hedge, the house's own shadow — that's no guarantee the same thing happens in summer, when the sun stands so much higher that it often clears the very same obstacle.

That makes the winter solstice the single most informative point to check if you only have time to investigate one moment closely. If you want the full picture — for example because you're considering planting a tree, building a shed, or judging where to put a terrace — you should look at several of the four dates, because different activities need sun at different times of year. A terrace you mainly use in summer needs sun most at the summer solstice; a winter garden needs sun most exactly when it's hardest to get.

How do you compare the seasons in practice?

That takes a program or model that can calculate the sun's position for a given date and time — not just show one fixed snapshot. Look for three things if you want to study a home thoroughly:

1. Can you pick any date at all, not just a single preset sun view?
2. Are the four key dates easy to jump straight to, so you don't have to type them in every time?
3. Does the calculation use the house's real location and north direction, or an assumed default? Without the right inputs, even a slick animation is only a guess — see the article on how the sun's path is calculated for the full explanation.

What HouseSense does

In the 3D model and the walkthrough in HouseSense, you can jump straight to the four seasonal turning points — both solstices and both equinoxes — or pick any date freely, and watch the shadows and light change in real time as you drag the time wheel. The calculation uses the house's own latitude, longitude and north direction, once they're set. If they're missing, the app falls back to a simplified sun arc and makes it clear that it isn't a precise calculation.

What the dates don't tell you

The sun calculation shows where the sun would sit on a cloudless day. It doesn't know the local weather — how many overcast days your area typically gets in December, or how thick the low-lying morning fog usually is. Nor does it show what physically blocks the sun from outside, unless that obstacle is part of the model itself. Use the four dates to understand the pattern of light across the year — not as a guarantee of how many hours of sunshine you'll actually get on a given day.