© NASA/Ricardo Nunes

Venus at greatest elongation west

Dominic Ford, Editor
From the Inner Planets feed

Objects: Venus
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Venus will reach its greatest separation from the Sun in its 2100 morning apparition. It will be shining brightly at mag -4.3.

From San Diego , this apparition will be reasonably placed and prominent, reaching a peak altitude of 40° above the horizon at sunrise on 17 Aug 2100.

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2100 morning apparition of Venus

12 May 2100 – Venus at inferior solar conjunction
15 Jun 2100 – Venus at greatest brightness
21 Jul 2100 – Venus at dichotomy
21 Jul 2100 – Venus at greatest elongation west
17 Aug 2100 – Venus at highest altitude in morning sky
02 Mar 2101 – Venus at superior solar conjunction

A graph of the angular separation of Venus from the Sun around the time of greatest elongation is available here.

Apparitions of Venus

24 Jul 2098 – Evening apparition
12 Dec 2098 – Morning apparition
01 Mar 2100 – Evening apparition
21 Jul 2100 – Morning apparition
05 Oct 2101 – Evening apparition
25 Feb 2102 – Morning apparition
12 May 2103 – Evening apparition

Observing Venus

Venus's orbit lies closer to the Sun than the Earth's, meaning it always appears close to the Sun and is lost in the Sun's glare much of the time.

It is observable for a few months each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

On these occasions, Venus is so bright and conspicuous that it becomes the third brightest object in the sky after the Sun and Moon. It is often called the morning star or the evening star.

These apparitions repeat roughly once every 1.6 years, taking place alternately in the morning and evening skies, depending whether Venus lies to the east of the Sun or to the west.

When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise.

At each apparition, Venus reaches a maximum separation from the Sun of around 48°. However, some times of the year are more favourable for viewing Venus than others. From San Diego, it reaches a peak altitude of between 27° and 47° above the horizon at sunrise during each morning apparition, depending on the time of year. During its 2100 apparition, it will peak at 40° above the horizon at sunrise on 17 Aug 2100.

This variability over the course of the year is due to the inclination of the ecliptic to the horizon.

The inclination of the ecliptic to the horizon

The inclination of the ecliptic to the horizon changes over the course of the year, affecting how high planets close to the Sun appear in the sky.

At all times, Venus lies close to a line across the sky called the ecliptic, which is shown in yellow in the planetarium above. This line traces the path that the Sun takes through the zodiacal constellations every year, and shows the plane of the Earth's orbit around the Sun. Since all the planets circle the Sun in almost exactly the same plane, it also closely follows the planes of the orbits of the other planets, too.

When Venus is widely separated from the Sun, it is separated from it along the line of the ecliptic. But, at different times of year, the ecliptic meets the horizon at different angles at sunrise. This means that Venus appears at different altitudes above the horizon at different times of year, even if its separation from the Sun is the same.

If the ecliptic meets the horizon at a shallow angle, then Venus has to be very widely separated from the Sun to appear much above the horizon. Conversely, if the ecliptic is almost perpendicular to the horizon, Venus may appear much higher in the sky, even if it is actually much closer to the Sun.

At sunset, the ecliptic makes its steepest angle to the horizon at the spring equinox – in March in the northern hemisphere, and in September in the southern hemisphere. Conversely, it meets the horizon at its shallowest angle at the autumn equinox. Because the seasons are opposite in the northern and southern hemispheres, a good apparition of Venus in one hemisphere will usually be poorly placed in the other.

At sunrise, these dates are also inverted, so that for morning apparitions of Venus, the ecliptic makes its steepest angle to the horizon at the autumn equinox, and its shallowest angle to the horizon at the spring equinox.

The optimum time for an apparition of Venus

The maximum altitude of Venus during all its morning apparitions between 2000 and 2050, as a function of the day of the year on which greatest western elongation occurs. Different colours show the altitudes observed from different latitudes. Click to expand.

For this reason, the day when Venus reaches its widest separation from the Sun (greatest elongation) is not necessarily the same day when it appears highest in the sky at sunrise. Venus typically appears highest in the sky a few days or weeks closer to the autumn equinox than the moment of greatest elongation.

The inclination of the ecliptic plane to the horizon at San Diego varies between 80° (sunrise at the autumn equinox) and 33° (sunrise at the spring equinox). On July 21, the ecliptic is inclined at 66° to the eastern dawn horizon, as shown by the yellow line in the planetarium above, meaning that this apparition of Venus will be reasonably placed and prominent, reaching a peak altitude of 40° above the horizon at sunrise.

Venus's position

The position of Venus when it reaches greatest elongation will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Venus 04h44m50s 18°48'N Taurus -4.3 23.5"
Sun 07h59m 20°34'N Gemini -26.7 31'28"

The coordinates above are given in J2000.0.

The sky on 28 Mar 2024

The sky on 28 March 2024
Sunrise
06:39
Sunset
19:05
Twilight ends
20:29
Twilight begins
05:15

18-day old moon
Waning Gibbous

85%

18 days old

Planets
Rise Culm. Set
Mercury 07:16 13:52 20:29
Venus 05:59 11:49 17:39
Moon 21:33 02:59 08:18
Mars 05:14 10:48 16:21
Jupiter 08:32 15:17 22:02
Saturn 05:40 11:21 17:02
All times shown in PDT.

Source

The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

21 Jul 2100  –  Venus at greatest elongation west
17 Aug 2100  –  Venus at highest altitude in morning sky
10 Jul 2101  –  Venus at highest altitude in evening sky
05 Oct 2101  –  Venus at greatest elongation east

Image credit

© NASA/Ricardo Nunes

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32.72°N
117.16°W
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