Ask Ethan: Why is it darker during winter than summer?

If you take a look outside these days, if you live where most of Earth’s humans do — in the northern hemisphere — you’re likely to see something completely expected: how bright it is compared to six months ago, in the dead of winter. It’s not just that the days are longer and the nights are shorter, which is what’s seasonally true in the summer as opposed to winter, but there are many other ways that the summer is brighter than the winter. These include:

• the darkest part of the sky, as seen during the day, is less dark in the summer than in the winter,
• the sky, just after sunset, gets darker more quickly in the winter than in summer,
• and that the darkest part of the night, in winter, is both darker and lasts for much longer as compared to summer.

Sure, the Earth rotates on its axis, and whether your hemisphere (north or south) is tipped toward or away from the Sun determines whether it’s summer or winter. While that accounts for the extended daylight hours of summer and the shortened daylight hours of winter, that doesn’t fully answer this week’s question, which comes to us courtesy of Brian Greenberg, who wants to know:

“Why does it appear darker outside in the winter? I need the science behind what we see.”

Believe it or not, it really is darker in the winter than in the summer by all of these measures, and you can understand why simply by tracing the path of the Sun through the sky. Let’s go through the science of why winter really is darker than summer.

This composite image shows the Sun’s path during three different events: Summer solstice (top path), Winter solstice (bottom path), and equinox (central path). The bright Sun corresponds to noon on the equinox.

The basics of what you experience are encoded in this wide-angle timelapse photo, above. As the Earth spins on its axis, the Sun appears to:

• rise in the east,
• reach a maximum height overhead during noon,
• and then set in the west,

with those arcs all appearing higher and higher in the sky at lower latitudes, but lower and lower in the sky at higher altitudes.

If you’re north of the Arctic Circle (or south of the Antarctic Circle), the curves appear to flatten out and get so low on the horizon that the lowest path — the one corresponding to winter solstice — never crests the horizon, as there is no sunshine on those days. The uppermost path, correspondingly, never sets, as the summer solstice corresponds to 24 hours of continuous sunlight.

Of course, if you’re closer to the equator, such as just north of the Tropic of Cancer (or just south of the Tropic of Capricorn), you’ll see the Sun appear nearly perfectly overhead at noon on the summer solstice, while the Sun reaches much lower above the horizon during the winter solstice. From anywhere between the Tropics and the Circles — i.e., at mid-latitudes, where most of the world’s population lives — there’s a whopping 47° difference between the maximum (during summer solstice) and minimum (during winter solstice) heights of the Sun above the horizon.

The Earth, moving in its orbit around the Sun and spinning on its axis, appears to make a closed, unchanging, elliptical orbit. The amount of daylight at any location on Earth depends on both the latitude of the observer and the tilt of Earth’s axis relative to the Sun.

Yes, this is due to our axial tilt. But what you might not realize is that, as the Earth spins about its axis, the apparent path of the Sun is highly dependent on your latitude. If you look at the upper diagram, to the right is Earth’s orientation during winter solstice from the northern hemisphere’s perspective, while to the left is Earth’s orientation 6 months later: during summer solstice. Yes, if you live interior to the Arctic Circle — at northern latitudes greater than 66.5° — you’ll never see the Sun during the winter solstice whereas you’ll always see the Sun during summer solstice.

But what happens if you’re south of the Arctic Circle, but not yet at equatorial latitudes?

Because the Earth is tilted by 23.5° relative to the Earth’s orbital plane around the Sun, you can see for yourself that the Sun always appears to make an “arc” through the sky as Earth rotates, but that arc is just part of a great circle: a full 360° circle around the Earth. The only catch is that the “night” hours occur wherever the Earth itself is in the way of that great circle.

During the summer, from the northern hemisphere, the Sun rises in the northeast, passes high overhead (toward the south), and sets in the northwest, and it’s night only when the Sun completes that arc during the brief time where the Earth is in the way. During the winter, however, the Sun appears to rise in the southeast, passes low above the horizon in the south, and sets in the southwest, while the entire rest of its 360° journey takes place at night.

This diagram shows the path of the Sun during Summer solstice (top arc) and Winter solstice (bottom arc) from a location at 50 degrees latitude. Note how the Sun traces out a much larger and longer path through the sky during the Summer Solstice, and dips below the horizon much more slowly during Summer than Winter.

This diagram, above, shows those different paths for someone who lives at 50° latitude. Just from this diagram, we can immediately understand the first way we experience darkness: the darkest part of the sky during daytime hours.

The reason our sky appears blue is because sunlight — itself composed of all the different colors of the spectrum — gets scattered by the particles present in the atmosphere. Blue (shorter-wavelength) light scatters more easily than red light, and so gets sent across the entire sky more easily than red light. If you look out toward the horizon during the day, however, you’ll notice that the sky appears a lighter shade of blue where the sky appears to meet the ground than toward the zenith, directly overhead. Directly overhead, instead, is where the shade of blue appears darkest.

That’s because there’s literally less atmosphere in the overhead direction as opposed to toward the horizon. Earth’s atmosphere is very thin compared to the actual size of the Earth: it forms a layer just 100-200 km thick, compared to a planet that’s more than 12,700 km in diameter. When you look up directly overhead, that’s all the atmosphere you see: that 100-200 km of it. But when you look closer to the horizon, there’s more atmosphere there, which means more of that scattered “blue” sunlight appears there, making the sky appear lighter in color.

Regardless of where the Sun is in the sky, the color of the sky toward the zenith (directly overhead) is a much darker blue, while the sky toward the horizon is a lighter, brighter cyan color. This is due to the larger amount of atmosphere, and the larger amount of scattered light, that is visible at low angles on the sky.

But now, let’s fold in what we know, additionally, about the Sun’s position in summer (near the summer solstice) versus in winter (near the winter solstice): it’s going to be an impressive 47° higher in the sky, closer to the directly-overhead zenith, in Summer as compared to winter. That means it’s:

• closer to being directly overhead,
• where it shines light more directly over where the traditional “darkest part of the sky” is,
• which means it brightens the entire sky more severely,

when it’s closer to the summer solstice than to the winter solstice. Even taking away the other factors, such as that it’s normally less cloudy and more clear in summer than in winter, this is enough to explain why the days are brighter in summer than in winter: the higher the Sun is in the sky, the brighter it appears.

You may also notice, even counting the moments from when the Sun first sets until it gets completely dark outside, that it takes longer for the sky to darken — not only completely but even severely — and for the stars to come out at night, during the summer as compared to the winter. There’s a scientific reason for that, too.

When the Sun just barely drops below the horizon, the sky is nearly still as bright as it was during daylight hours. As it descends below the horizon, we go through civil, nautical, and astronomical twilight before the onset of night, and then the twilight happens in reverse when the new day arrives.

The various levels of darkness we experience at night, after the Sun has set, depend primarily on one and only one factor: how low the Sun actually is below the horizon. Although we classify the time period between the complete darkness of night and either the rising or setting of the Sun as twilight, there are actually three different types of twilight that we experience here on Earth.

• There’s civil twilight, which occurs just before sunrise, at dawn, or just after sunset, at dusk, where the Sun is nearest to the horizon but just below it: between 0° and 6° below the horizon. Artificial lighting is typically unnecessary, as the remnant solar illumination suffices for the human eye. Typically, only Venus, the brightest planet, is visible in the night sky.
• There’s nautical twilight, which is the next stage: when the Sun is between 6° and 12° below the horizon. The brightest stars and all of the naked-eye planets are visible during this time, and human vision begins to require artificial illumination to see the details in most objects on Earth.
• And finally, there’s astronomical twilight, which occurs when the Sun is between 12° and 18° below the horizon. All bright stars as well as most intermediate and many fainter stars become visible to human vision, but most extended objects (such as nebulae and galaxies) as well as the faintest naked-eye stars of all remain invisible.

It isn’t until the Sun exceeds 18° below the horizon that true “darkness” is achieved.

This world map shows the latitude dependence of what types of twilight are achievable during the solstices: the June solstice for the northern hemisphere and the December solstice for the southern hemisphere. Above 48 degrees latitude, the true darkness of night never arrives during the Summer solstice.

There’s something profound that we can learn almost immediately: because of Earth’s axial tilt, anyone who’s at higher latitudes than 48.5° will never experience the full darkness of true “night” on the day of the summer solstice, as the Sun will never get more than 18° below the horizon. This problem gets more and more severe at higher latitudes, as:

• above 54.5°, astronomical twilight is never achieved on the summer solstice,
• above 60.5°, even nautical twilight is never achieved on the summer solstice,
• and above 66.5°, there is no twilight at all on the summer solstice, just 24 hours of continuous daylight.

However, even at latitudes that are more modest, such as in the 20s, 30s, and 40s of degrees either north or south of the equator, the duration of these stages of twilight last for much longer near the summer solstice than the winter solstice. Even though the darkest part of the night is still equally dark regardless of the seasons, the duration of darkness is not only shorter during the summer than winter, it takes longer after sunset (and longer before sunrise) for the Earth to transition through the various stages of twilight.

During the Summer solstice, the great circle that the Sun traces out in the sky spends relatively little time below the horizon, and takes longer to dip below the 18 degree threshold than during either equinox or during the Winter solstice. This explains the more rapid onset of night in the Winter, and the slower onset of night in the Summer.

Take a look at the diagram above, shown again for a location that’s at 50° (north) latitude. As you can see, when we compare the two solstices with the equinox, we find that the Sun reaches a lower minimum below the horizon during winter than during summer. In particular, for this example:

• the Sun reaches 16.5° below the horizon at summer solstice,
• the Sun reaches 40° below the horizon at the equinoxes,
• and the Sun reaches 63.5° below the horizon at winter solstice,

where the 23.5° tilt of our planet is responsible for these severe differences. Clearly, there are more hours where the Sun is 18° or more below the horizon near the winter solstice than near the equinoxes, and also more hours where that’s true near the equinoxes than near the summer solstice.

But there’s something else to realize here: the farther away from the equator you are on planet Earth, the “flatter” these circles appear. This means that the higher your latitude is and the closer you are to the summer solstice, the more time the Sun spends in the twilight phase and the less time the Sun spends in the “night” phase. (Similarly, the Sun spends less time in twilight and more time in the night phase near winter solstice.)

As the more severely curved paths for the Sun’s trajectory below the horizon illustrate, as seen more pronouncedly at higher altitudes (50 rather than 40) and closer to the Summer solstice (as opposed to equinox or Winter solstice), the Sun is slower to move through the “twilight” phases and reach the true darkness of night.

Again, take a look at the above diagram, which shows not only the Sun’s path through the sky during daylight hours at two different latitudes, but also the “invisible” path as it dips below the horizon.

Look, in particular, at how the Sun’s path “flattens out” more during the summer solstice (high path) as compared to either the equinoxes (middle path) or the winter solstice (bottom path). Also note how this effect is more severe at higher latitudes: 50° vs. 40°. Even this relatively small difference is substantial near the summer solstice!

This becomes even more clear if we don’t project the sky into a full 360° panorama, but rather divide the day and night up evenly by time, and look at the Sun’s apparent path as viewed from Earth during the course of a day. You can see, even not at the solstice, how the first 6° below the horizon, marking civil twilight, elapse more quickly (when the Sun is not near its peak depth below the horizon) than either nautical twilight, which marks the next 6° (from 6° to 12°), or than astronomical twilight, which marks the last 6° (12° to 18°) before the onset of true darkness at night. Look at the diagram below, which shows the durations of these various stages at an altitude of 53° North, corresponding to Brighton, England, over a 24-hour interval from August 12 to August 13: about 8 weeks after the summer solstice.

As the Sun sets on August 12 from a latitude of 53 degrees north, civil twilight lasts for 37 minutes after sunset, nautical twilight lasts for the next 49 minutes, and astronomical twilight endures for a further 58 minutes, before just 4 hours and 32 minutes of true nighttime darkness. Then the reverse happens: astronomical dawn lasts for 57 minutes, nautical dawn lasts for 48 minutes, and civil dawn persists for 38 minutes. The “curved” apparent path of the Sun is the reason for this.

So there you have it. It really is brighter during the summer than the winter, not only in the simple terms of daylight hours versus nighttime hours, which is dictated by the Earth’s axial tilt and how it’s oriented with respect to the Sun, but it’s “brighter” in three additional profound ways.

1. The daytime is actually brighter in summer not just because it’s sunnier, but because the height of the Sun in the sky illuminates what’s typically the darkest part of the sky, directly overhead, more significantly than in the winter.
2. The nights are longer in winter than the summer, and in particular the amount of time that the Sun spends below that critical 18° threshold with respect to the horizon, is more severely longer in the winter as opposed to the summer.
3. And the amount of time it takes the sky to darken, through civil, nautical, and astronomical twilight before reaching the true darkness of night is longer in the summer than in the winter, with a corresponding amount of additional time required for the sky to brighten through astronomical, nautical, and then civil dawn during the summer as well.

The summer really is brighter, and the winter really is darker, for everyone on Earth who lives outside of the equatorial tropics. The farther away from the equator you go, the more severe it gets, with Earth’s orbit and axial tilt being the primary reason behind it all.

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