The Venus Evening Apparition of 2010

by Martin J. Powell

Following superior conjunction on January 11th 2010 (when it passed directly behind the Sun) Venus emerged in the dusk sky in late February 2010 as an 'Evening Star', setting in the West shortly after the Sun. As typifies a Venusian evening apparition, the planet was slow to emerge from the twilight glow, taking several weeks to gain a significant altitude (angle above the horizon) after sunset. When seen through a telescope, the planet showed a broad gibbous phase at this time, shining at an apparent magnitude of -3.9 and measuring only around 10" across (i.e. 10 arcseconds, where 1" = 1/60th of an arcminute or 1/3600 of a degree). However, its low altitude, great distance from the Earth and small apparent size made it a difficult object to observe telescopically, with no detail being discernible in its clouds. The planet was positioned on the Aquarius/Pisces border as it entered the evening sky, moving steadily North-eastwards along the ecliptic (the apparent path of the Sun, Moon and planets) at a rate of about 1°.2 per day.

On March 12th the planet made a brief exit from the zodiac, entering the constellation of Cetus, the Whale before re-entering Pisces along its Southern border on March 15th. On March 9th Venus crossed the celestial equator (declination = 0°) heading Northwards and three weeks later (on March 30th) it entered the constellation of Aries, the Ram. The planet passed a wide 11° to the South of that constellation's brightest star Hamal ( Arietis, mag. +0.2) on April 4th.

As March gave way to April, Venus became more readily visible to Northern hemisphere observers, hanging low over the Western horizon and setting an hour or so after sunset. Now at a more comfortable solar elongation of around 20°, the brilliant-white planet began to catch the attention of millions of people worldwide as they made their homeward journey from workplaces, colleges and schools. Viewed from the Southern hemisphere, the planet did not really become prominent until around mid-May.

Venus remained in Aries for a little under three weeks, entering Taurus, the Bull on April 19th. Five days later (April 24th) the planet passed 3°.6 South of the open star cluster known as the Pleiades (pronounced 'PLY-add-eez' or 'PLEE-add-eez') or The Seven Sisters (M45 or Messier 45), perhaps the best-known star cluster in the night sky. Now sinking into the evening twilight, the cluster is not seen at its best at this time of year. Under dark skies the seven brightest stars in the group can be seen with the naked-eye and they are often considered to be a good test of visual acuity; they are Alcyone ( Tauri, mag. +2.9), Atlas (mag. +3.6), Electra (mag. +3.7), Merope (mag. +4.2), Taygete (mag. +4.3), Pleione (mag. +5.1v) and Celaeno (mag. +5.5).

On May 4th Venus passed 6°.5 North of Taurus' brightest star Aldebaran ( Tauri, mag. +0.9), a red giant star which appears as part of the V-shaped open cluster of stars known as the Hyades. The cluster comprises around 200 stars spread over an area of about 5° of the sky; they lie at a distance of about 150 light years from Earth (where 1 light year = 63,240 Astronomical Units). The presence of Aldebaran within the cluster is purely a line-of-sight effect; in fact the star is not a physical member of the group, being at a much closer distance of 68 light years.

For observers at high-Northern latitudes, the month of May saw Venus at its highest altitude in the sky after sunset for the 2010 apparition, the planet being around 15° high some 30 minutes after sunset. This is because, from these latitudes, the ecliptic presented a high angle to the Western horizon at the point in the zodiac where Venus was positioned at the time (namely, in Taurus and Gemini). Although the angle of the ecliptic to the horizon may be steep, the solar elongation of 30° around this time was not as wide as other evening apparitions of Venus where the planet is viewed under similar circumstances. In late March 2012 and early February 2017, for example, the angle of the ecliptic to the horizon was also steep after sunset but Venus was then positioned over 40° from the Sun and consequently was much higher in the sky (see the accompanying table showing the maximum altitudes attained by Venus after sunset during its 8-year cycle).

Venus left Taurus and entered Gemini on May 20th, and shortly afterwards (May 23rd) the planet attained its highest declination (angle above the celestial equator) for this apparition of +25°.0. Venus then set at its most Northerly point along the horizon, an effect which is more pronounced the further North in latitude an observer is situated.  For example, at the Equator (latitude 0°) the planet set in the West-North-west at this time whilst at 60° North the planet set in the North-North-west, some 35° further North along the horizon.

By early June, Venus had brightened a little to magnitude -4.0 and with a phase of around 80% was appearing notably gibbous through telescopes, having enlarged in size to around 13 arcsconds. On June 7th the planet passed 8°.2 South of Castor ( Geminorum, mag. +1.6), Gemini's second-brightest star. Two days later (June 9th) the planet passed 4°.7 South of Gemini's brightest star (and Castor's twin) named Pollux ( Geminorum, mag. +1.1). Southern hemisphere observers, now approaching local Winter, began to see Venus against the backdrop of a truly dark sky, the planet shining like a distant arc-lamp above the North-western horizon and setting over two hours after the Sun.

Venus entered Cancer - the faintest of the zodiac constellations - on June 12th. Around this time, observers at mid-Northern latitudes saw Venus reaching its highest altitude in the sky after sunset for the 2010 apparition, the planet being positioned around 20° to 25° above the horizon some 30 minutes after sunset.

On June 20th the planet passed 0°.8 North of the star cluster commonly known as Praesepe (pronounced 'pree-SEE-pee') or the Beehive Cluster. Under dark, rural skies it is visible to the naked-eye as a hazy patch of light and in city locations it is easily seen in binoculars. The cluster is also referred to by its more technical names M44 and NGC 2632.

Venus moved from Cancer into Leo on June 29th, where it remained for a little under five weeks; the planet first appears on the star chart (top of page) from about July 5th. On July 10th the planet passed 1°.1 North of Leo's leading star Regulus ( Leonis, mag. +1.3), which is at the base of the Sickle of Leo, an asterism shaped like a backwards question-mark [].

In late July and into August, observers situated in Equatorial latitudes saw the planet reaching around 35° altitude at 30 minutes after sunset, the highest altitude of the 2010 apparition attained by the planet from these latitudes.

Now brightening more significantly, Venus moved from Leo into Virgo on August 1st, shining at magnitude -4.2. On August 10th Venus was in conjunction with Saturn (i.e. the two planets attained the same celestial longitude), Venus passing 3°.1 to the South of the ringed planet. Saturn was now considerably more distant from the Earth and consequently dimmer than it had appeared when at opposition in the previous March, having faded from magnitude +0.5 to + 1.2 (for more details see the Saturn 2006-13 page).

From mid- to late August, observers at mid-Southern latitudes got their chance to see Venus at its highest altitude in the sky after sunset. In fact, from these latitudes the planet reached a higher altitude in the sky after sunset than any other latitude during the 2010 apparition, the planet being some 38° above the horizon at 30 minutes after sunset.

Venus reached its greatest Eastern elongation from the Sun on August 20th, when it was positioned 46° East of the Sun and shone at magnitude -4.3. At this point, telescopes showed Venus resembling a small half-Moon (phase = 0.50 or 50% illuminated) with an apparent diameter of  24".4. Whilst the time of greatest elongation is normally considered to be the best time to observe the planet, in this particular apparition observers at high-Northern latitudes found the planet to be low down in the sky after sunset and caught in the long summer twilight, being visible for only an hour or so after sundown. Unlike the situation three months earlier, Venus' position in South-western Virgo meant that the planet was now on the descending (Southward) section of the ecliptic. Seen from high-Northern latitudes, the ecliptic now presented a shallow angle to the local horizon after sunset, positioning the planet low down in the sky (telescopic observers of the planet would be less concerned about this, since they normally observe the planet in full daylight, when it is much higher in the sky). Southern hemisphere observers fared much better at this stage of the apparition, Venus having been high up in the sky and visible for over three hours after sunset.

For a few days around greatest elongation, telescopic observers often look to find the precise moment when the terminator (the line seperating the light and dark sides of the planet) appears perfectly straight, essentially dividing Venus in two perfect halves. In theory this should occur on greatest elongation day, however it often does not and the precise reason for this is not known. This anomaly is known as Schröter's Effect and it is well-known amongst telescopic observers of the planet.

On August 23rd, Venus passed 3°.1 South of Mars, the Red Planet now an unremarkable pale-orange 'star' shining at magnitude +1.5. Mars had been slowly fading since its aphelic opposition in late January and had been moving steadily Eastwards along the zodiac at a rate of about 0°.5 per day since mid-March (for more details, see the Mars 2009-10 page).

Although reaching its greatest elongation angle (46°) on August 20th, Venus was in fact positioned at 46° from the Sun for a three-week period from August 8th through to August 31st. From early September the planet began to close in towards the Sun once more, its solar elongation now reducing, with telescopes showing a crescent phase and a rapidly enlarging apparent disk size. On September 1st, Venus passed 1°.0 South of the star Spica ( Virginis, mag. +1.0), the brightest star in Virgo. During September the apparent size of the Venusian disk enlarged from 28".3 (on September 1st) to 44".0 (on September 30th). Having started the month at greatest elongation distance, Venus ended the month some 9° closer to the Sun.

On September 23rd, whilst approaching the Virgo/Libra border, Venus attained its greatest brilliancy for this apparition at magnitude -4.5. The planet's greatest brilliancy occurs when the percentage of the illuminated portion of the disk (phase) and its angular size combine to best visual effect. In 2010 this took place when the planet was 25% illuminated (phase = 0.25), its angular diameter was 39".8 and its solar elongation was 39°. For observers at high-Northern latitudes, however, the planet's greatest brilliancy in 2010 was not seen against a truly dark sky because the planet was then low down after sunset and masked by twilight. Southern hemisphere observers, on the other hand, saw Venus in its true majestic brilliance in a dark sky above the Western horizon.

When Venus entered Virgo in early August it was moving at a daily angular motion of 1°.0, but by the time it entered Libra on September 24th its motion had slowed to 0°.5 per day and its path through the sky had deviated Southward, carrying it some 6° away from the ecliptic. Mars, moving at a more steady rate through Virgo and keeping closer to the ecliptic, effectively 'caught up' with Venus, the two reaching conjunction for a second time on September 29th.

Venus reached a stationary point in South-western Libra on October 8th, at which point its prolonged period of direct (West to East) motion ceased and it commenced retrograde (East to West) motion, turning back towards Virgo and re-entering that constellation on October 20th. Now heading directly into the Sun's glare, the planet's period of visibility from Earth was drawing to a close.

From around early October, steadily-held binoculars detected the crescent of Venus soon after sunset, as the planet languished low in the West-South-western sky. Telescopes showed a large, thin crescent at this point, the image greatly disturbed by turbulence in the Earth's atmosphere and split into the rainbow colours by an effect called dispersion (an example of how dispersion appears through a telescope can be seen here).

As the Venusian crescent continued to enlarge it also became more slender, such that the dark (non-illuminated) side of the planet was well-displayed when seen from the Earth. With the aid of ultraviolet and infrared filters, telescopic observers now began their search for the mysterious and elusive Ashen Light, a faint glowing of the night side of Venus which until recently had no clear explanation. Given the extremely high temperatures which are known to exist beneath the Venusian clouds, the Ashen Light is today considered by many to be a visual indication of the planet's surface glowing red hot.

From mid-October, observers with exceptionally-good eyesight may have attempted to observe the crescent of Venus with the naked-eye. Whilst this may seem extraordinary, the planet's apparent size of around 58" brings it very close to the generally-accepted resolution of the human eye, i.e. 1' (1 arcminute, or 1/60th of a degree). Because the planet's solar elongation was then around 15° or less, glare was no longer a problem because the planet was then seen in bright twilight through to its setting, theoretically allowing the crescent to be discerned more easily.

During the fnal week of October Venus quickly sank into the twilight glare and became lost from view from all locations, reaching inferior conjunction (passing between the Earth and the Sun) on October 29th. The period of non-visibility was brief, however; by the second week of November 2010, Venus was seen rising as a 'Morning Star' in the Eastern sky shortly before the Sun, heralding a new morning apparition (2010-11) which lasted through to July 2011.

 [Terms in yellow italics are explained in greater detail in an associated article describing planetary movements in the night sky.]

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Moon near Venus Dates, March to October 2010

The Moon is easy to find, and on one or two days in each month, it passes Venus in the sky. Use the following tables to see on which dates the Moon passed near the planet between March and October 2010:

The Moon moves relatively quickly against the background stars in an Eastward direction, at about its own angular width (0º.5) each hour (about 12º.2 per day). Because it is relatively close to the Earth, an effect called parallax causes it to appear in a slightly different position (against the background stars) when seen from any two locations on the globe at any given instant; the further apart the locations, the greater the Moon's apparent displacement against the background stars. Therefore, for any given date and time listed in the table, the Moon will have appeared closer to Venus when seen from some locations than others. For this reason, the dates shown in the table should be used only for general guidance.

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Direction, Altitude & Visibility Duration of Venus after Sunset, March to October 2010

The following tables give the direction and altitude (angle above the horizon) of Venus at 30 minutes after sunset, together with the visibility duration of the planet after sunset, for the 2010 evening apparition. An explanation of abbreviations in the tables is given in the box below. For the sake of convenience, the table is split into Northern and Southern hemisphere latitudes (the Equator is included in both tables to allow interpolation of the data for observers situated at Equatorial latitudes). These tables should have proved sufficient to locate the planet in twilight, allowing telescope users to view the planet in comfort (because of Venus' brilliance, glare becomes a problem when the planet is seen through the eyepiece against a dark sky). Direction and Altitude diagrams are also provided below for intermediate latitudes of 55° North, 35° North, 30° South and the Equator.

The tables allow one to find the highest altitude in the sky which Venus attained for any given latitude during the 2010 evening apparition, and in which direction it was seen. For example, observers situated at latitude 40° North (a mid-Northern latitude) found the planet highest in the sky (at 30 minutes after sunset) in early June, when it was seen at an altitude of 21° towards the West-North-west. Around this time, the planet was above the horizon for about 2½ hours after sunset.

Northern Hemisphere Latitudes

Southern Hemisphere Latitudes

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Direction & Altitude Diagrams (Horizon Diagrams) for the 2010 Evening Apparition

The following diagrams show an observer's Western horizon (from due South to due North) for latitudes of 55° North (a high-Northern latitude), 35° North (mid-Northern), the Equator and 30° South (mid-Southern). The path of Venus is plotted in the sky at 30 minutes after local sunset throughout the 2010 evening apparition with the planet's direction and altitude marked along the horizontal and vertical axes, respectively. Essentially, these diagrams show the same information as in the above look-up tables, but in an illustrative format, for the Equator and three intermediate latitudes.

For higher accuracy, the azimuth (the bearing measured clockwise from True North) is also shown along the direction axis. For each of the latitudes shown, the direction and altitude of Venus after sunset can be estimated for any part of the 2010 evening apparition by positioning your pointing device over each image, when an overlay grid will appear, marked at 10° intervals; the values can then be read off accordingly.

Although the dates indicated in the above diagrams refer specifically to the year 2010, Venus has an 8-year cycle of apparitions such that its position in the evening sky in 2010 repeated very closely in the evening sky of 2018. The writer refers to this particular evening apparition as Apparition A; for more details, see the accompanying article describing The Venus 8-year Cycle.

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Naked-eye Venus: Apparitions, Conjunctions and Elongations

Credits

• The star chart was created using by the commercial software MegaStar (by Willmann-Bell, Inc). Star colours were based on those shown in "Patrick Moore's Colour Star Atlas" (Mitchell Beazley Publishers Ltd., London, 1973).
• The dates of the planet's constellation boundary crossings were determined from MegaStar and the daily rates of angular motion were derived from a BASIC computer program ('PLANLONG.BAS') written by the writer.
• The directions, altitudes and visibility durations of Venus were derived from the commercial software RedShift 5 by Maris Technologies Ltd.
• The times and separations of the lunar conjunctions with Venus (Moon near Venus dates) and the lunar phases were obtained from RedShift 5 using the 'Conjunctions' option of the 'Sky Diary'.

• The zodiac signs are courtesy of the free graphics site .

Copyright  Martin J Powell  April 2010