The physicist Fred Hoyle predicted in 1948 that once a photograph of the entire earth was taken from outer space “a new idea as powerful as any in history will be let loose.” His words were prophetic. NASA’s famous “Blue Marble” shot, taken on the final Apollo manned moon mission in 1972, created a worldwide sensation when it was released. Nearly 50 years later, it remains the only image taken by astronauts of the entire globe and is believed to be one of the most widely reproduced photographs of all time. Our brilliantly illuminated planet, pictured looking up at Antarctica and the continent of Africa from below, does indeed appear like a giant blue glass marble set off against the inky blackness of space.

The photo op had not been part of the original mission plan. The shot was taken five hours into the flight, with the Apollo 17 spacecraft positioned so the entire planet was lit up as it sped toward the moon. Astronaut Jack Schmitt* seized the opportunity to take the earth’s picture with a modified 70-mm Hasselblad – the only camera not stowed away for use later in the flight.

The curious thing is that the photo was published upside down. You would never know it to look at the image, because Antarctica appears at the bottom of the picture where you would expect it. However, the spacecraft was oriented in such a way that the South Pole was actually at the top of the world and the Arabian peninsula at the bottom when the picture was taken. The photographer was 28,000 miles from home in zero gravity and would have had no bodily sense of up and down. There is, of course, no particular reason why north should always be up and south down. Presumably NASA decided to adhere to earthly convention so as not to confuse people.

Humans have been mapping our world for some 16,000 years, but only in last few centuries has north been consistently positioned at the top. For example, although Christopher Columbus navigated by the North Star when he set sail for the New World, he thought of east as being the top of the world because he believed that was the direction of paradise. North only took top honors starting with the Mercator map in 1569, which attempted to account for the curvature of the earth on a two-dimensional surface. The map was especially useful for navigation, most of which took place in the northern hemisphere at that time, so it made sense to orient the map that way.

To what extent is our basic orientation in space and time simply a matter of convention? Granted, “up” and “down” are arbitrary designations if you are looking at a map. But is there some fundamental alignment of direction with the sun and planets? The answer is yes and no. The planets all orbit the sun on the same plane in the direction of its rotation: counterclockwise when viewed from the sun’s north pole. And the planets rotate on their axes in this same direction, with the notable exception of Venus and of Uranus. Uranus rotates on its side, possibly because it got knocked off its pins after colliding with an earth-sized object long ago. A collision may also explain why Venus’ north and south poles are essentially the reverse of most other planets relative to the plane of the Solar System. The so-called ecliptic, which is defined by the orbits of the planets around the sun, is at a 63° angle to the Milky Way, and the universe as a whole appears to be a complete jumble. If you whipped out your Hasselblad to take a 360° picture of the entire universe, there is no way you could determine which way is up and which is down.

The best we can do is to orient ourselves in relation to particular objects, whether landmarks on earth or heavenly bodies beyond it. The same is true of time. When Isaac Newton laid out the basic framework of the universe in the 17th century, he assumed time worked like some giant invisible clock on the back wall of the universe, ticking away. Einstein later maintained time was strictly local, speeding up or slowing down according to the velocity of an observer relative to a given frame of reference.

As a practical matter, we mark time by the movements of the sun and moon in the sky. We calibrate the days by the rotation of the earth on its axis and the years by the earth’s orbit around the sun, with the phases of the moon sometimes thrown in for good measure. If we lived on Mars, things would be more or less the same, with the days slightly shorter and years nearly twice as long and two moons to keep track of as well. Uranus is another matter entirely. The planet rotates on its axis every 17 hours but is tilted on its side, orbiting the sun every 84 years. As a result, the northern hemisphere is bathed in sunlight constantly during a summer season lasting 21 earth-years and in darkness for another 21 years during the winter. In between, day and night are determined by Uranus’ 17-hour sideways rotation on its axis.

How would you mark time if you had grown up on Uranus? A baby born in the northern hemisphere at the start of summer would be old enough to order a drink by the time the sun set. Tracking the phases of the moon – or rather moons, all 27 of them -- would be impossible during the years of uninterrupted sunlight and next to impossible to keep track of the rest of the time. Our sense of time passing, or duration, depends on the regular recurrence of certain phenomena, such as sunrises, sunsets, full moons and equinoxes, which is why most ancient civilizations had a circular conception of time.

Suppose there were no suns, moons, planets or stars in the sky. Would we have any conception of time? Certainly, we would be able to observe the passing of events, but would there be any way to measure their progress? Change and duration are not the same thing. In order for an event to have duration, we must be able to mark its passing, if only to put a stick in the ground to follow the sun’s shadow as the day advances. Otherwise, everything just happens right now, and although we might have memories of other “nows,” we would have no way to measure the distance between them.

While this may sound farfetched, we know there is no clock ticking on a back wall of the universe. The rooster needs the sun to come up before it can crow. Hunter-gatherer tribes had no concept of linear time apart from events, nor did they need to, because they were not farmers who had to schedule planting and harvesting based on cycles of nature. The Amazonian Amondawa tribe, which was discovered in 1986, had no words for time or increments of time, such as “month” or “year.” The Hopi language likewise has no words for time or even a grammatical structure that segments actions into past, present and future. Time for the Hopis is a purely subjective experience of things getting later.

If there is a temporal direction to the universe, it is that things get later. They go from seed to harvest, from spark to cinder, and not the other way around. But that doesn’t mean there is a clock running the whole time. Experiments have shown that subjects isolated underground without clocks quickly lose an accurate sense of time passing. Something similar happens in cases of severe retrograde amnesia, when you can no longer remember what happens from one moment to the next, and it is effectively always right now. Conversely, as I enter my eighth decade, I am continually astonished to discover that memories from long ago seem like only yesterday; indeed, I am finding that a lifetime can go by in practically no time at all.


*Schmitt’s fellow Apollo 17 astronauts, Eugene Cernan and Ron Evans, also claimed to have taken the shot, but evidence suggests Schmitt, a geologist by training, was most likely the photographer.

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