Venus in our Sky and our Telescopes

Because Venus orbits inside the orbit of Earth, the planet always stays relatively close to the Sun, but at times Venus can become easily observable. After the planet passes behind the Sun, a position called superior conjunction the planet will slowly begin to appear in the evening sky. At this time, the Sun is traveling about one degree per day across the sky eastward against the background of the stars, or 30 degrees per month. Venus at this point is traveling 38 degrees eastward per month, so the planet only very slowly pulls away from the Sun. By a month after superior conjunction the planet has pulled only seven or eight degrees ahead of the Sun. At that pace, it usually takes two to three months for Venus to pull far enough away from the Sun to become easily visible. How long it takes to actually begin to see the planet as it rises above the horizon depends not only on angular distance from the Sun but also upon the angle of the ecliptic relative to the horizon, as we discussed during the chapter one observing projects. If it is springtime in the north, the planet will become visible much more rapidly than it would during autumn. If you can train a telescope on the planet, it shows you a tiny disk barely ten arc seconds across, nearly fully illuminated. As the months go on, it will become obvious that Venus is becoming more and more gibbous in shape, exactly as a waning Moon does. As Venus circles the Sun, it slowly begins to present more and more of its dark side towards Earth. As the planet begins to grow larger in telescopes, it also grows brighter. From magnitude -3.3 at superior conjunction the planet will surpass magnitude -4 by the end of the fifth month of the apparition. The planet's phase continues to wane as is draws closer to Earth. By five months after superior conjunction, the planet has grown to about 17 arc seconds in diameter and has waned to about two-thirds illuminated. Now Venus begins "rounding the corner" in its orbit, traveling less tangentially to our line of sight and more directly towards us. This causes the planet's eastward motion to slow gradually. By the time the planet is eight months past superior conjunction, Venus has swelled to 24 arc seconds in size and it reaches a position in space where it is now at the corner of a right angle with Earth and Sun. At this time, Venus is at its maximum angular distance from the Sun. This value ranges from between 46 and 48 degrees depending whether Venus is at perihelion or aphelion at the time. With the right angle geometry,Venus at this point shows us exactly a half illuminated disk, a condition called dichotomy. Now Venus' rate of eastward motion against the stars slows to less than that of the Sun and the Sun begins to overtake it in the sky. The halfphase rapidly begins to wane to a crescent as the planet moves ever more rapidly towards the Sun. Eventually the planet's eastward motion ceases and it begins to move westward on the sky, an effect called retrograde motion. The planet continues to swell, wane and brighten until about 38 days prior to passing between Earth and the Sun, a point called inferior conjunction. At this point Venus is about 38% illuminated and spans about 38 arc seconds. At this time the planet achieves its maximum brightness or what astronomers refer to as greatest brilliancy. At magnitude -4.8, Venus shines so brightly in the evening sky that it can cast shadows on the ground. The planet's brightness is dependent entirely on how much surface area is visible to us. Until now, the planet has been growing in size faster than its waning phase can diminish the total surface area. So at this time, Venus shows us the largest amount of surface area expressed in square arc seconds and thus achieves its maximum brightness. This is the time of greatest illuminated extent and this is when an inferior planet is at its brightest. But from this point forward, the planet's waning phase begins to diminish the total surface area faster than its rapidly growing size can make up for. Over the final month of the apparition Venus begins to fade and fall from the sky, closing on the Sun at faster than a degree per day. The planet's crescent phase fades to a hairline even as it lengthens to as much as a full arc minute. The planet's brightness fades from its peak to about -3.6. Before long, the planet is completely lost from view. How long you can keep it in sight will again depend upon the angle of the ecliptic relative to the horizon. Venus's orbital inclination can also be a factor. Although the planet's orbit is inclined only a few degrees from Earth's, the difference is enough that if Venus is on the north side of its orbit, it can add quite a bit of altitude to the planet's position and delay its disappearance by a few days. But in the end, disappear it will as it overtakes Earth and passes between the Sun and us.

As Venus moves rapidly to the west of the Sun, it will begin to reappear in the morning sky as fast as it fell from the evening sky and thus become what the ancients called the "morning star." Venus runs through a morning apparition in exactly the reverse order that it ran through an evening apparition. The planet appears within a few days or weeks, depending upon the angle the ecliptic makes with the morning horizon. Late summer and early autumn offer the most favorable conditions. Just over a month after inferior conjunction, the planet achieves greatest brilliancy and a month later the planet's westward motion stops and direct motion resumes. About ten weeks after inferior conjunction, the planet reaches greatest elongation west of the Sun and then again its eastward motion against the stars causes it to again begin to overtake the Sun. Over the next eight months, the planet slowly closes the gap between it and the Sun, waxing from dichotomy at greatest elongation to a gibbous phase that grows more and more full as the planet shrinks in size back to a minimum of about nine or ten arc seconds by the time the planet reaches superior conjunction again about twenty months after the last time it passed behind the Sun.

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