VARYING VIEWS OF VENUS
With two extremely dazzling planets giving us company for the next seven months, a long series of exciting times are ahead for all who care to raise their heads slightly upwards in the sky soon after sunset. The bright beacons lighting the western and eastern skies are Venus and Jupiter. Venus is preparing itself to show us plenty of attractive sights over the coming months.
Venus is currently in the western sky just after sunset, and appears higher as the days pass. Jupiter is also rising in the east at sunset, so both planets will be higher in the sky with passing days. The difference between them is how their positions change in the hours after sunset. Venus drops lower in the west while Jupiter rises higher in the east; as the Earth turns on its axis with passing hours.
The positions of the two planets will change day by day, when observed at the same time, over several days. This change depends on the Earth’s orbit around the Sun. As days pass, we are able to see different stars at night because we see the Universe from ever changing directions.
Since Jupiter is a very distant planet (more than 5 times as distant from the Sun as Earth) and moves slowly in its orbit (nearly 12 years to complete one orbit), its position in space is practically constant. However, as the Earth shifts its position in its orbit, Venus will be seen earlier and earlier each day. Hence, that planet will rise higher and higher at each sunset, reaching the zenith (overhead point) at the end of the year.
For Venus, the story is very different. First, the planet is only about three quarter as far from the Sun as Earth. That is, its orbit is inside that of Earth’s. Sometimes it is very close to the Earth when it is on the same side of the Sun as the Earth (called inferior conjunction).
When it is on the other side of the Sun (i.e. at superior conjunction) Venus is much farther (nearly six times away) from the Earth. The planet moves relatively rapidly in its orbit, taking only 225 days, about two thirds of a year, and so overtakes Earth every 584 days. This causes the position of Venus in the sky to shift visibly day by day.
Since the orbit of Venus is inside that of Earth, it can never be seen overhead at night. The maximum angle that it can attain in the sky at sunset is about 47 degrees. The size of Venus is similar to that of Earth, and because its distance changes so significantly, its apparent size (as seen through a telescope) changes almost six-fold between its most distant and its closest approach to Earth.
Another consequence of the orbit of Venus being inside that of our Earth is that Venus shows changes in phases similar to the changes we see in the shape of our Moon. In fact, if one is not careful, one can easily be fooled into thinking that the Venus crescent you see in the telescope is that of the Moon.
The changing view of Venus over the next seven months is shown in the diagram together with the planet’s phases on different dates during its path. Venus’ march in the western skies began low in the western horizon in October. At that time, it was in superior conjunction, on the farther side of the Sun from Earth. During November, its path will turn left, rising higher each night. When seen through a telescope at this time, it will appear as a tiny, almost circular dot.
In the next stage of movement during December and January next year, it will veer right, rising to a maximum elevation of 35 degrees at sunset. Its phase in this stage is referred to as the gibbous phase, appearing in the telescope as a slightly bigger dot with one edge cut off.
From February to April 2012, it will move faster almost horizontally, being at almost the same elevation throughout during these four months. Through a telescope it will appear slightly larger, but in half phase.
Over the month of May, it will begin rapidly dipping lower and lower in the west, with its apparent size increasing dramatically to more than five times larger that when it first started its journey in October. Through a telescope, its phase will become an increasingly thinner crescent. Though the crescent becomes thinner, the planet shines with the same brightness, since its apparent size increases as the planet comes closer to Earth.
The planet will come close to the Sun by the end of May and will disappear below the horizon with the setting Sun. In fact by then, Venus will be on its way to a historic meeting with the Sun on 6th of June 2012, when it will cross the face of the sun, known scientifically as the Transit of Venus, an event that will not be repeated in our life time ,since the next transit will occur after more than a century in 2117. We will be able to witness the transit of 2012 while the Sun rises on June 6.
During the shift of Venus in the western sky over the coming seven months, there will be several interesting close encounters with other planets, stars, and the Moon. Since the beginning of this month, another inner orbit planet, Mercury, the smallest and nearest planet to the Sun, has been closing in on brilliant Venus. In the sky, you will see a tiny dot which is Mercury rising up each day getting closer to Venus. On Sunday November 6, the two will be closest, appearing at the same level about a finger width apart. The pair are in the mouth of Scorpio and are closing in on its brightest red supergiant star Antares, to which the pair will be closest (i.e. in conjunction) on November 10, when they will appear in a line and present a dramatic view.
By this time, Venus will have left Mercury below and continued to climb in the skies. The meaning of the word planets as “wanderers” becomes even more vivid when you see that by the end of November and beginning December, Venus will have moved from Scorpio to the constellation Sagittarius, a movement of nearly 30 degrees across the sky. By noticing this change in positions of the planets in the evening sky, you are witnessing the actual movement of the planets in space.
On 26 November, Mercury and Venus will be joined by a one-day-old crescent after the New Moon of the previous day. The thin crescent Moon will be in between, with Mercury below and Venus above it. This should be a remarkable sight to view in the low western skies and well worth witnessing. On the next day, the spectacle continues when a still thin crescent will be higher in the sky, with Venus and Mercury below it.
Mercury being an inner planet with an orbit inside that of Earth and Venus, shows changes in phases as seen from Earth. During the time from November 10 to the end of November, Mercury will be lower and lower in the sky and its phase will change from half to crescent. This is a very good opportunity for those with telescopes to view this elusive planet and get a rare view of its crescent phases.
Over the coming months you will read more tidbits about other encounters and interesting spectacles that Venus will amaze us with, so, keep your interest alive.