Space is black—so why is the sky blue…and the sunset red?

It has to do with the properties of sunlight and Earth’s atmosphere.

Sunlight is made up of many wavelengths of visible light. Combined, we see them as white. But individually they’re a rainbow of colors, from violet and blue to orange and red.

When sunlight enters Earth’s atmosphere, it collides with molecules of oxygen, nitrogen, and other particulates, which scatter the light into different colors.

Violet and blue light have the shortest frequencies and the most energy, so they scatter the most. Much of the violet light is absorbed by the upper atmosphere, but blue light is reflected throughout the atmosphere, giving us a blue sky.

Once near the surface, blue light has been scattered and reflected so much, it recombines with other colors into full-spectrum white light.

When the sun nears the horizon, sunlight passes through much more atmosphere. That longer journey scatters and eventually filters out more of the blue light, leaving the longer-wavelength orange and red light to reach our eyes.

But it’s not this way everywhere. On Mars, the atmosphere is mostly CO2 and filled with iron-rich dust, which scatters the red light, turning the sky there…red.

Except at sunset, when the longer trip through the Martian atmosphere scatters and filters so much red light that blue light passes through—making the sunset…blue!

You may remember from an earlier EarthDate that seawater is so salty, drinking it pulls water from the rest of your body, which can lead to dehydration and even death.

To keep their ships stocked with fresh water, sailors have relied on innovation and technology for centuries.

Early on, they realized they could funnel rainwater from their sails into storage—once the rain had washed away the ocean spray.

Greek sailors then discovered they could hang sheep pelts in the cool night air to absorb water vapor, then wring them out in the morning for a wool-flavored drink.

The earliest European sailors used barrels laced with alcohol to keep algae from growing.

Whenever a ship reached land, replenishing its fresh water was usually the most important task.

By the 1700s, inventors had created distillation plants that used a heat source to boil seawater. When it converted to steam, it left the salt behind. The vapor would then condense again into pure distilled water.

Distillers were used until 1980, when they were replaced by reverse-osmosis systems that use membranes to purify and desalinate water.

Today, navy fleets of about 50 ships have the capacity to desalinate more than a million gallons of water a day.

Cities with large naval bases, like San Diego, are looking at ways to use their fleets’ desalination capacity to supplement their municipal water systems in times of drought and other emergencies.

Think of Earth as a rubber ball. If you push in at one place, the surface will indent. Surrounding that indentation, the ball will bulge outward.

Near the end of the last Ice Age, just 20,000 years ago, continental ice sheets covered 25 percent of Earth’s landmass, pushing down on Earth’s surface with as much as 150 tons per square foot of pressure.

This depressed the surface and pushed up the land around the perimeter.

During the last 15,000 years, the Ice Age ended and glaciers have naturally retreated to cover only 11 percent of the land surface today.

And just like a rubber ball, the Earth’s surface has been rebounding—very slowly.

Areas that were depressed by ice are rising. Canada, Scotland, and Scandinavia all have risen 4 to 6 inches over the past century.

Areas that had bulged outward at the edge of glaciers are now sinking, back to pre–Ice Age levels, around 4 inches per century.

This can be seen in southern England and Ireland and the northern and middle U.S., including many cities like Chicago, Milwaukee, and Cleveland.

In low-lying near-coastal cities like London and Washington, D.C., falling land levels combined with rising seas from continued glacial melt are making those cities more prone to flooding.

We can expect rising and falling landmasses, and associated shoreline changes, as Earth continues its slow motion Ice Age rebound.

Within a tiny salmon egg, molecules of magnetite are already gathering.

Once the egg hatches and the salmon begins to grow, it moves downriver, eating larger prey, taking in iron from its environment and its food, to synthesize more magnetite to store in its body—particularly in the sinus bones within its skull.

Once it’s large enough to enter the ocean, its scales change from green and brown to the silver and gray of the open sea.

The magnetite in its nose begins to line up into chains that can detect and respond to the magnetic fields of the Earth.

The salmon is developing its magnetoreception.

In the ocean, it feeds on fish and krill, ingesting more iron, storing more magnetite, traveling thousands of miles—up to 18 miles a day—over the next few years, guided in the dark waters by its three-dimensional magnetoreception, sensing not only direction but intensity and inclination of the magnetic field.

When it’s time to return to its home river, magnetoreception is aided by another sense in its nose: smell.

The salmon can detect just a few parts per million of its birth river in ocean currents and follow them home. Once there, it will mate and die.

Nutrients and minerals from its body will return to the stream to nourish future salmon, who will make their own magnetite for their own magnetoreception, to guide their own miraculous journey, handed down across thousands of generations.

What makes leaves change colors so dramatically in the fall?

Three things: leaf pigments, the weather, and the length of the days, called the photoperiod.

To protect themselves from freezing, broad-leafed trees in temperate areas must harvest the sugar from their leaves for the winter.

This starts when shorter days signal trees to slow the production of chlorophyll.

As photosynthesis uses up the remaining green chlorophyll, yellow pigments that are always present in the leaves show through.

Mild sunny fall days will rapidly process the chlorophyll and leave bright golds, while rainy or hot days will make for more muted colors.

Cooler nights soon trigger the production of red and purple pigments, which are thought to act as a sunscreen, further slowing photosynthesis.

These red colors are more abundant in healthier plants and may serve to warn insects away, toward weaker plants.

Eventually, the last sugars are drawn from the leaves and into the branches, trunk, and roots of the tree for storage during the winter.

Cells form at the base of the leaf, making it more likely to fall off, and at the twig end, like a scab, sealing the twig off from outside elements.

Only the vascular bundles connected to the veins of the leaf hold it to the tree.

When the leaf finally falls, its remaining nutrients are recycled into the soil to be used by the tree for future growth.

Meanwhile the bundle scar left on each twig becomes a bud for a new leaf in the spring—when the tree will use its stored energy to grow a new crown.

The sun may appear constant but, like most things in the universe, it’s constantly in flux—cycling between an active state, called a solar maximum, and a more restful one, called a solar minimum.

It alternates regularly between these about every 11 years.

During solar maxima, sunspots increase but so do solar flares, making these times of increased irradiance.

Higher-than-normal electromagnetic radiation causes geomagnetic storms on Earth that impact GPS systems, satellites, and power grids.

Charged particles emitted can be hazardous to astronauts and spacecraft electronics.

More X-rays and extreme UV radiate toward Earth, but most are absorbed by the outer atmosphere, called the thermosphere.

The increased energy causes the thermosphere to expand, interfering with the orbit of satellites.

Conversely, during solar minima, solar flares and charged particle emissions are rare.

This is generally good news, since it means fewer geomagnetic storms and lower solar radiation risk.

However, there’s less solar energy to form ozone in the middle atmosphere, which allows more sunburn-producing UVB to reach Earth’s surface.

And reduced solar wind means that more cosmic rays can reach Earth.

Both solar states pose unique challenges. But for the most part, our atmosphere reacts to the increasing or decreasing energy and continues to protect us.

The most abundant element, making up an incredible 75 percent of the known mass of the universe, is hydrogen. It also might be the energy fuel of the future.

For more than 4 billion years, hydrogen has fueled our sun, a nuclear fusion reactor that fuses hydrogen into helium to light up our solar system.

Hydrogen has also fueled our rockets. It’s very lightweight, a huge advantage for space travel. Under extreme pressure, or at extremely low temperatures, it becomes liquid.

Without a spark and an oxidizer, liquid hydrogen is stable. So rockets also carry liquid oxygen and an ignition source. Once lit, hydrogen packs a serious punch, burning at more than 5,000 degrees Fahrenheit.

Hydrogen may one day power our earthbound transportation, too. In most fuel cells, hydrogen is forced through a membrane. It splits off an electron as electricity, which can run an electric motor to propel a vehicle.

To fill the tank of a fuel-cell car, hydrogen has to be highly compressed—but in that state, it contains three times the energy, pound for pound, as gasoline.

And when processed through a fuel cell, the only emissions are water vapor.

On Earth, hydrogen doesn’t often exist in a pure gas form. But it can be separated from water by electric current.

Whether used in cars, rockets, or even fusion reactors, this essential element could, and probably should, play an ever-more-important role in powering the world.

The human body is amazingly capable…despite the fact that, in many places, it’s engineered all wrong.

Take for example, our lower leg. Our foot has 26 bones, and our ankle, 7. Far more moving parts than needed to walk or run, making us much more prone to injury.

How did this happen?

Well, we evolved from earlier hominins who evaded predators and found food in the trees.

Those ancestors needed a grasping foot, like a chimpanzee’s—with a big toe like an opposable thumb and a flexible ankle like a wrist.

As we climbed down from the trees and onto land, our feet had to evolve.

The foot and ankle became rigid to make a propulsive lever. The arch developed as a shock absorber. The big toe moved forward in line with the others.

This worked well enough. Even though they’re built on an old structure more like hands, our flawed feet carried us into the modern day.

Now, look down at your feet and imagine a different design: a streamlined lower leg wrapped in ligaments, not muscles; a fused ankle ending in one or two large, simple toes with just one bone in each.

Sounds weird—but this is the structure that propels ostriches and horses to speeds over 40 miles an hour, absorbing more stress while using comparatively less energy.

Different structures, borne from different evolutionary pressures.

We’ll look at other curious cases of human evolution in future EarthDates.

Earlier on EarthDate, we talked about the three North Poles.

Of these, geographic north, the tip of Earth’s axis, defines latitude and longitude on our maps, while magnetic north is what compass needles point toward.

Unfortunately, geographic north and magnetic north are not in the same place. So the north of our maps and the north of our compasses don’t match.

The difference between them is called declination.

To complicate matters, declination varies depending on your location. Compass navigations have to be offset by the declination of that particular spot to make them accurate to the map.

Now for the real curveball: magnetic north is always moving!

It’s based on Earth’s magnetic field, which is created by turbulent currents in Earth’s molten metal outer core as they swirl around its solid iron inner core.

Shifting currents mean the magnetic field is always moving. Which means magnetic north is always changing. And declinations around the world are always adjusting!

We can see this effect all around us. Streets and walls built to compass north a few centuries ago may now be off by several degrees.

Runways, which are named by their compass orientation to help pilots navigate them, periodically have to be renamed when the declination of that spot changes.

To keep planes and ships safe, scientists track the movements of magnetic north and the constantly changing declinations and keep them updated every 5 years.

It might surprise you to know there are actually three North Poles: the geographic, the magnetic, and the geomagnetic.

The one we most commonly think about is the Geographic North Pole—the tip of the axis that the globe spins around. It’s in the middle of the Arctic Sea, on pack ice, above more than 2 miles of water.

During the summer, when the pole is tilted toward the sun, it’s 24-hour daylight. In the fall, the sun finally sets—once, and only once, per year. That brings on 6 months of night, until springtime, when the once-annual sunrise starts another 6 months of day.

The North Magnetic Pole is slightly to the south. This is where Earth’s magnetic field, which springs out of the ground at the South Magnetic Pole, dives back into the ground.

Its exact location, however, is constantly changing, because Earth’s magnetic field is always changing.

Today, magnetic north is moving about 34 miles a year, gradually traveling toward Siberia.

Finally, there’s the North Geomagnetic Pole—the northern axis of the magnetosphere, the magnetic shield that protects us from solar winds.

If you imagine the magnetosphere as a ball around a bar magnet, with the bar running through Earth, this pole would be the positive end. It’s currently 700 miles south of the Geographic North Pole but also always traveling.

With three dark places to look in the dead of winter, and two of them moving, it’s a wonder Santa ever makes it home.