APOD: Phases of Venus (2020 May 21)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4035
Joined: Fri Dec 04, 2009 3:27 am

APOD: Phases of Venus (2020 May 21)

Post by APOD Robot » Thu May 21, 2020 4:06 am

Image Phases of Venus

Explanation: Just as the Moon goes through phases, Venus' visible sunlit hemisphere waxes and wanes. This composite of backyard telescopic images illustrates the steady changes for Venus during its current stint as our evening star, as the inner planet grows larger but narrows to a thin crescent. Images from bottom to top were taken during 2020 on dates February 27, March 20, April 14, April 24, May 8, and May 14. Gliding along its interior orbit between Earth and Sun, Venus grows larger during that period because it is approaching planet Earth. Its crescent narrows, though, as Venus swings closer to our line-of-sight to the Sun. Closest to the Earth-Sun line but passing about 1/2 degree north of the Sun on June 3, Venus will reach a (non-judgmental) inferior conjunction. Soon after, Venus will shine clearly above the eastern horizon in predawn skies as planet Earth's morning star. After sunset tonight look for Venus above the western horizon and you can also spot elusive innermost planet Mercury.

<< Previous APOD This Day in APOD Next APOD >>

De58te
Science Officer
Posts: 313
Joined: Mon Sep 30, 2013 6:35 pm

Re: APOD: Phases of Venus (2020 May 21)

Post by De58te » Thu May 21, 2020 10:49 am

This picture reminds us just how ingrained we are about thinking that in astrophotography that North is at the top of the photo. Presuming that where Venus is lit up that the Sun would be above the picture, yet the description says that Venus will be 1/2 degree north of the Sun, so then it is South at the top of the photo and North is at the bottom. Am I wrong?

User avatar
orin stepanek
Plutopian
Posts: 5690
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: Phases of Venus (2020 May 21)

Post by orin stepanek » Thu May 21, 2020 11:09 am

Oh Venus; no planet shall come between us! :mrgreen:
Orin

Smile today; tomorrow's another day!

GeoXXXXX

Re: APOD: Phases of Venus (2020 May 21)

Post by GeoXXXXX » Thu May 21, 2020 1:03 pm

This is a wonderful chance to view Venus and Mercury extremely close together relative high in the sky.

If you need to get your bearings here they ate an hour before sunset
Image

This shot is just after sunset a little zoomed in to approximate the naked eye view.Notice how high they are riding in the sky, both are over 15 degrees above the horizon. That is about the width of two fists (thumbs tucked in) at arms length. Venus is currently getting closer to the Sun from our view point every day while Mercury is still going up!
Image

A closeup showing the distance between them, only a little more than two full moons! You can see them both at once through a pair of binoculars! Not too often you can do that!
Image

If you want to test your eyesight this is a great chance to do it. Venus is about 55 arc seconds across right now. While nominal human eyesight can detect objects as a non-point source at about 60 arc-seconds size (Other modifiers includes contrast and other factors as well) people with superb eyesight can make it out as a crescent 🌙. Give it a try!

BTW Don’t waste your time trying to perceive Mercury’s disc, it’ only 6 arc-seconds across!

Please forgive me if I have butchered the explanations in this post!

Eric

GeoXXXXX

Re: APOD: Phases of Venus (2020 May 21)

Post by GeoXXXXX » Thu May 21, 2020 1:07 pm

De58te wrote:
Thu May 21, 2020 10:49 am
This picture reminds us just how ingrained we are about thinking that in astrophotography that North is at the top of the photo. Presuming that where Venus is lit up that the Sun would be above the picture, yet the description says that Venus will be 1/2 degree north of the Sun, so then it is South at the top of the photo and North is at the bottom. Am I wrong?
Actually North is on the left, they laid Venus on it’s side!

Sa Ji Tario

Re: APOD: Phases of Venus (2020 May 21)

Post by Sa Ji Tario » Thu May 21, 2020 1:42 pm

In her book Astronomy, Cecilia Payne Gaposchkin says that the light reflected by Venus and captured on Earth always gives the same intensity value. Observe that as the planet gets closer to its visible area it is larger but its illuminated area drops, which causes the intensity of received light to always have the same value.-
Some of this has already been said by José Comas Solá when referring to the inner planets, in his book Astronomía (by chance of identical names, only that in Comas Solá's illustrations are drawings, therefore, older)

User avatar
neufer
Vacationer at Tralfamadore
Posts: 17420
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Phases of Venus (2020 May 21)

Post by neufer » Thu May 21, 2020 1:57 pm

https://www.etymonline.com/search?q=phase wrote:
phase (n.) 1705, "phase of the moon, particular recurrent appearance presented by the moon (or Mercury or Venus) at a particular time," back-formed as a singular from Modern Latin phases, plural of phasis, from Greek phasis "appearance" (of a star), "phase" (of the moon), from stem of phainein "to show, to make appear".

Latin singular phasis was used in English from 1660 for each of the aspects of the moon. General (non-lunar) sense of "aspect, appearance, stage of development at a particular time" is attested by 1841. Meaning "temporary difficult period" (especially in reference to adolescents) is attested from 1913.
Art Neuendorffer

User avatar
Chris Peterson
Abominable Snowman
Posts: 15247
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Phases of Venus (2020 May 21)

Post by Chris Peterson » Thu May 21, 2020 2:03 pm

Sa Ji Tario wrote:
Thu May 21, 2020 1:42 pm
In her book Astronomy, Cecilia Payne Gaposchkin says that the light reflected by Venus and captured on Earth always gives the same intensity value. Observe that as the planet gets closer to its visible area it is larger but its illuminated area drops, which causes the intensity of received light to always have the same value.-
Some of this has already been said by José Comas Solá when referring to the inner planets, in his book Astronomía (by chance of identical names, only that in Comas Solá's illustrations are drawings, therefore, older)
That's approximately true. We generally consider Venus to have an apparent magnitude of -4. In actuality it varies from -3.7 to -4.5 as its distance and phase change. Visually that's not a big difference, but it is quite apparent to an observer paying attention.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
johnnydeep
Science Officer
Posts: 193
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Phases of Venus (2020 May 21)

Post by johnnydeep » Thu May 21, 2020 2:51 pm

Hmm. I note that all these crescents are horizontal. Does that mean something particular about where the pictures were taken on the Earth? Is a horizontal crescent mandatory (or even possible) for an inferior planet when viewed from Earth? Or was this just depicted this way for artistic effect? Sorry if the answer should be obvious, but I always have trouble visualizing planetary geometry in 3D :-(
"To Boldly Go......Beyond The Fields We Know."

User avatar
Chris Peterson
Abominable Snowman
Posts: 15247
Joined: Wed Jan 31, 2007 11:13 pm
Location: Guffey, Colorado, USA

Re: APOD: Phases of Venus (2020 May 21)

Post by Chris Peterson » Thu May 21, 2020 3:03 pm

johnnydeep wrote:
Thu May 21, 2020 2:51 pm
Hmm. I note that all these crescents are horizontal. Does that mean something particular about where the pictures were taken on the Earth? Is a horizontal crescent mandatory (or even possible) for an inferior planet when viewed from Earth? Or was this just depicted this way for artistic effect? Sorry if the answer should be obvious, but I always have trouble visualizing planetary geometry in 3D :-(
The images have been aligned this way as an artistic choice. Venus has only a small axial tilt, so its crescent phases basically define north and south with respect to the ecliptic. The apparent angle we see it at (with respect to the horizon) depends on our latitude and where the planet is in the sky.
Chris

*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

User avatar
johnnydeep
Science Officer
Posts: 193
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Phases of Venus (2020 May 21)

Post by johnnydeep » Thu May 21, 2020 4:00 pm

Chris Peterson wrote:
Thu May 21, 2020 3:03 pm
johnnydeep wrote:
Thu May 21, 2020 2:51 pm
Hmm. I note that all these crescents are horizontal. Does that mean something particular about where the pictures were taken on the Earth? Is a horizontal crescent mandatory (or even possible) for an inferior planet when viewed from Earth? Or was this just depicted this way for artistic effect? Sorry if the answer should be obvious, but I always have trouble visualizing planetary geometry in 3D :-(
The images have been aligned this way as an artistic choice. Venus has only a small axial tilt, so its crescent phases basically define north and south with respect to the ecliptic. The apparent angle we see it at (with respect to the horizon) depends on our latitude and where the planet is in the sky.
Ok, that makes sense to me - thanks!
"To Boldly Go......Beyond The Fields We Know."

Sa Ji Tario

Re: APOD: Phases of Venus (2020 May 21)

Post by Sa Ji Tario » Thu May 21, 2020 4:42 pm

Cris, thanks for the clarification

User avatar
MarkBour
Subtle Signal
Posts: 1073
Joined: Mon Aug 26, 2013 2:44 pm
Location: Illinois, USA

Re: APOD: Phases of Venus (2020 May 21)

Post by MarkBour » Thu May 21, 2020 5:19 pm

That's a lovely composition and gives a very helpful comparison of the progression.
Thanks for that nice work, Richard Addis!
(And thanks to our APOD astronomers and curators for finding and sharing this.)

Capture.JPG
It might be fun to lay out these images on a depiction of the planetary orbits.

Something like the (very inaccurate) rendering at the right:
You do not have the required permissions to view the files attached to this post.
Mark Goldfain

User avatar
neufer
Vacationer at Tralfamadore
Posts: 17420
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Phases of Venus (2020 May 21)

Post by neufer » Thu May 21, 2020 6:59 pm

MarkBour wrote:
Thu May 21, 2020 5:19 pm


That's a lovely composition and gives a very helpful comparison of the progression.

(And thanks to our APOD astronomers and curators for finding and sharing this.)

It might be fun to lay out these images on a depiction of the planetary orbits.

Something like the (very inaccurate) rendering at the right:
The synodic period between inferior conjunctions = 583.92 Earth days:

Almost exactly the same as 5 solar Venusian solar days = 583.75 Earth days.

Hence, every time Venus is at inferior conjunction,
Earth observes the same nighttime side of Venus.*


That nighttime side of Venus even slowly rotates retrograde
....as if it is constantly tracking Earth.


Venus seems to be very very leery of the Earth :!:
-----------------------------------------------------------------------------------
* Over the long 4-transit Venus cycle [of 243 years]
there will be 152 inferior conjunctions during which
we will observe Venus to rotate only ~80º in latitude.
Art Neuendorffer

User avatar
Ann
4725 Å
Posts: 10676
Joined: Sat May 29, 2010 5:33 am

Re: APOD: Phases of Venus (2020 May 21)

Post by Ann » Thu May 21, 2020 7:00 pm

MarkBour wrote:
Thu May 21, 2020 5:19 pm
That's a lovely composition and gives a very helpful comparison of the progression.
Thanks for that nice work, Richard Addis!
(And thanks to our APOD astronomers and curators for finding and sharing this.)

Capture.JPG
It might be fun to lay out these images on a depiction of the planetary orbits.

Something like the (very inaccurate) rendering at the right:
Thanks, Mark, that's beautiful! :D

Ann
Color Commentator

TheZuke!
Science Officer
Posts: 178
Joined: Fri Mar 22, 2019 2:19 pm

Re: APOD: Phases of Venus (2020 May 21)

Post by TheZuke! » Tue May 26, 2020 2:23 pm

Thanks MarkBour and neufer for the illustrations!