If I want to figure out the next date and time when Venus will be the furthest east of the sun-- for purposes of knowing when it will be highest in the sky the greatest number of hours after sunset (for best viewing)-- What is the criteria I should really be looking at?
One. Venus' greatest elongation angle east of the Sun?
Two. The greatest difference in Ecliptic Longitude between Venus and the Sun?
or
Three. The greatest difference in hours of Right Ascension along the equatorial grid between Venus and the Sun?
Thanks,
-- John
Last edit: B3 Burner 2016-08-20
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Venus' Greatest Elongations range from about 45°-47°, its RA elongations range from about 42°-49°.
Around the equator that means Venus will set about 3 hours after the sun sets. Outside the equator in terms of viewability and how long after sunset Venus sets these RA / Elongation differences don't matter much. What does matter and is the biggest factor is Venus' declination. That is if Venus is above, the same or below the zodiac compared to the sun.
If Venus is above the sun (like in 1/2017 & 3/2020), it will be best seen in the Northern Hemisphere and not so good in the south, when Venus is below the sun (like 8/2018 & 10/2021), it's best seen in the Southern Hemisphere and no so good in the north.
For example, the Venus' next eastern greatest elongation in Jan 2017, the Sun is in Sagittarius while Venus is 13° above it in Aquarius. So in the Northern Hemisphere at mid-latitudes (~25°-45°) Venus will set about 4 hours after sunset while in the Southern hemisphere it will set only 2 hours after sunset. The reverse will be true in 8/2018 when the sun is in Leo and Venus is below in Virgo.
Or seasonally, Venus (and Mercury) best eastern elongations always occur in winter and spring of whatever hemisphere you are in. Likewise, the reverse is true of the Western/Morning Elongations when they are best seen in summer and fall.
Summer has a longer dusk, so once you get go above or below 45° N or S latitude seeing Venus at Eastern GE during summer becomes very difficult.
In some cases, like in 6/2023 Venus can end up a similar declination as the sun (Venus hits the top of the zodiac and starts to come down the other side). In those cases Venus will set about 3 hours after sunset in both hemispheres.
If I want to figure out the next date and time when Venus will be the furthest east of the sun-- for purposes of knowing when it will be highest in the sky the greatest number of hours after sunset (for best viewing)-- What is the criteria I should really be looking at?
One. Venus' greatest elongation angle east of the Sun?
Two. The greatest difference in Ecliptic Longitude between Venus and the Sun?
or
Three. The greatest difference in hours of Right Ascension along the equatorial grid between Venus and the Sun?
Thanks,
-- John
Last edit: B3 Burner 2016-08-20
Venus' Greatest Elongations range from about 45°-47°, its RA elongations range from about 42°-49°.
Around the equator that means Venus will set about 3 hours after the sun sets. Outside the equator in terms of viewability and how long after sunset Venus sets these RA / Elongation differences don't matter much. What does matter and is the biggest factor is Venus' declination. That is if Venus is above, the same or below the zodiac compared to the sun.
If Venus is above the sun (like in 1/2017 & 3/2020), it will be best seen in the Northern Hemisphere and not so good in the south, when Venus is below the sun (like 8/2018 & 10/2021), it's best seen in the Southern Hemisphere and no so good in the north.
For example, the Venus' next eastern greatest elongation in Jan 2017, the Sun is in Sagittarius while Venus is 13° above it in Aquarius. So in the Northern Hemisphere at mid-latitudes (~25°-45°) Venus will set about 4 hours after sunset while in the Southern hemisphere it will set only 2 hours after sunset. The reverse will be true in 8/2018 when the sun is in Leo and Venus is below in Virgo.
Or seasonally, Venus (and Mercury) best eastern elongations always occur in winter and spring of whatever hemisphere you are in. Likewise, the reverse is true of the Western/Morning Elongations when they are best seen in summer and fall.
Summer has a longer dusk, so once you get go above or below 45° N or S latitude seeing Venus at Eastern GE during summer becomes very difficult.
In some cases, like in 6/2023 Venus can end up a similar declination as the sun (Venus hits the top of the zodiac and starts to come down the other side). In those cases Venus will set about 3 hours after sunset in both hemispheres.
See my Planetary Events scripts
http://www.lowerwolfjaw.com/astro/Stellarium/pev.htm
They demonstrate Venus' Greatest elongations (There's a current repeating pattern of 5 Eastern GEs every 8 years)
qam1: Thank you for your thoughtful reply. That was quite helpful. Sorry it took me two weeks to see your response.
-- John