Closer look at different phases of Venus


Some of life’s simple pleasures cannot be altered by the stress of a global pandemic.

Even the most casual stargazer will notice the planet Venus at some time over the next couple of months. The second planet from the sun shines a blazingly brilliant white.

Look for it low in the southeast during bright morning twilight. It’s hard to miss. Only the sun and moon shine more brightly.

That tiny dot of light hides many secrets. A small telescope or a good pair of binoculars reveals that the lit portion of the planet looks like a thin crescent right now.

Like the moon, Venus has phases, which shouldn’t be surprising. As Earth and Venus orbit the sun, the sun’s light falls on our sister planet at different angles from our point of view.

Our moon takes about 30 days to complete a set of phases. Venus does so every 584 days.

Observe Venus with a telescope or binoculars during the next two months as it undergoes a remarkable change in appearance.

Its apparent size and shape are determined by the relative positions of Venus and your earthly vantage as both planets orbit the sun. Venus is closer to the sun and travels faster around it.

Venus thus orbits the sun once every 225 days. Earth does so every 365 or so days. Venus is constantly pulling away from or careening toward Earth as it travels at 21.8 miles per second, and we poke along at 18.5.

The effect is an odd one. Venus travels through space against the starry background. However, she will seem to hold her position close to the horizon in the east during evening twilight for the next few weeks.

Over two months, Venus grows from a 24-percent-illuminated crescent up to about a “half” Venus as it sinks slowly toward the horizon.

One might expect a half Venus to be brighter than a crescent Venus.

However, because Venus has the inside track in the race around the sun, as the weeks pass, Venus will get a bit farther from Earth and thus decrease in its angular diameter. As its illuminated part gets bigger, its apparent size gets smaller, and it appears dimmer in the sky.

Right now, crescent Venus is about 35 million miles from Earth.

As the illuminated part of Venus increases, so will its distance. On March 28, the planet will be almost 70 million miles away from us, about double the distance from Earth.

Consequently, at the end of March, Venus will appear to have shrunk to about half its size on Feb. 8. (For you astronerds: From 43.5 arcseconds tomorrow to about 21 arc seconds at the end of March.)

The upshot: the half Venus of late March will be dimmer than the crescent Venus of early February.

The most beautiful views will occur during the mornings of late March.

During March, the ruddy planet Mars will slowly rise, day by day, to take its place to the right of Venus.

By the end of March, Saturn will have risen high enough to be visible below Venus. On March 28, a thin crescent moon, just 16 percent illuminated, will join the celestial tableau below Mars.

Venus illustrates both the value of science and its limitations.

For most of human history, the phases of Venus were hidden in the data that every human being who looked at Venus could easily see.

Its brilliant white light shines so beautifully that the Greeks and Romans worshiped the planet as the goddess of love and beauty.

The belief in the goddess Venus died slowly, partly because the planet is just so spectacular to look at.

As science began to replace faith in the old gods, Earth remained the center of the cosmos. Common sense tells us that the stars and planets revolve around the Earth. Come on. Go out and look.

The trouble was that an Earth-centered model could not accurately predict the exact positions of the planets in the sky.

It took a spectacular improvement in human technology to make the hidden information visible.

That revolutionary technology was, of course, the telescope. When Copernicus argued in the 1540s that Earth orbited the sun, he contradicted 1,000 years of common sense based on direct observation.

Copernicus put the sun at the center and made Earth just another planet orbiting the sun in a perfectly circular orbit.

Luckily for Copernicus, some Dutch lens makers discovered that distant objects could appear closer by arranging two lenses in just the right way. Over many months around 1609, the Italian astronomer Galileo used a crude telescope to observe the disk of Venus shrink and grow.

He concluded that such phases could not occur if the sun and Venus orbited Earth. Galileo argued the Copernican system must be correct. (He was wrong about that, but never mind.)

Copernicus didn’t imagine in his wildest dreams that Venus had phases, even though those phases are a logical consequence of his system. The data about the phases was so hidden in the Venusian light that it took a whole new way of seeing Venus to ferret it out.

Every time Copernicus looked at the planet, the proof was there, but he had no eyes to see it.

The process has been repeated countless times over the long course of human history. It’s as if the universe is constructed of many layers of undeciphered code, each more complex than the next.

The pursuit of knowledge is an extended intelligence test. We must invent increasingly clever ways of reading more and more complex code if we are ever to understand the way things really are.

With all its emphasis on proof and logic, science must have faith in one unprovable proposition. Again and again, some new technology will come along to dislodge more hidden information. Until that happens, we humans don’t even know the next great question, let alone what the answer will be.

In more practical applications, the uncertainty of science can be troubling. How easy it is for us to simply believe the pronouncements of a political or religious leader, whose statements take on an emotional certainty even if they contradict the available evidence.

But we must beware of such pronouncements. In the case of COVID-19, they are laden with abstract, emotionally charged terms like “freedom,” “faith,” and “liberty.”

However, we must balance our personal freedoms against the pressing needs of society. “Liberty” does not mean that we can ignore the rights of others. As the old saying goes, “Your right to swing your arm ends at the tip on my nose.”

Science can be slow and halting. It can travel down many false paths before finding the right one, but it will undoubtedly find the right road in the end.

To paraphrase an old saying about democracy, science is the worst way of finding the truth — except for all the other ways.

Uncertainty is to be expected in times like these, which is why I love stargazing so much. Such glorious uncertainty is meat and potatoes for stargazers like me.

Stargazing thus becomes an odd combination of the systematically logical science of astronomy and the powerfully emotional experience of staring up with wondrous perplexity at the starry sky.

If your heart is pure and your mind is right, emotion and intellect merge to form a profound sense of your place in the universe.

Start by seeing Venus with your heart and not your mind. The planet is so startlingly bright and astonishingly white that you will see why our ancient forbears called her the goddess of love and beauty.

And then let the science of astronomy intrude. Sadly, astronomy provides contradictory evidence to Venus’s beautiful face and form. The planet’s brightness is merely reflected sunlight enlivened by the planet’s proximity and atmosphere.

Venus is a tiny, Earth-sized mirror at about 8,000 miles wide, but we are its next-door neighbors in the solar system. Venus reflects so prodigiously because of its unbroken layer of shiny sulfuric-acid clouds.

The planet’s acrid mist would melt the skin off the goddess’s face, given half a chance. Her radiant beauty is as thin as her atmospheric skin. Add to that a thick atmosphere of heavy carbon dioxide. If you stood on Venus, you would be crushed flat by the weight of the air above their heads.

The dense Venusian atmosphere acts like a blanket and holds in the heat generated by the sun. Add the Venusian gale-force winds, and everywhere you go you, would find a temperature of 900 degrees Fahrenheit.

So much for the goddess of love and beauty.

The eye will know where you and Venus are in space. The intellect will know precisely why. And the heart will soar at the intricate beauty of the motions.

Suddenly, you will be standing in a near infinity of space on a tiny, spinning ball of rock orbiting at breakneck speed around a star as another planet races by.

By Tom Burns


Tom Burns is the former director of the Perkins Observatory in Delaware.

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