APOD: Crescents of Venus (2023 Aug 26)

[APOD Robot]

Crescents of Venus

Explanation: Just as the Moon goes through phases, Venus' visible sunlit hemisphere waxes and wanes. This sequence of telescopic images illustrates the steady changes for Venus during its recent 2023 apparition as our evening star. 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 the inner planet swings closer to our line-of-sight to the Sun. Closest to the Earth-Sun line but passing about 8 degrees south of the Sun, on August 13 Venus reached its (non-judgmental) inferior conjunction. And now Venus shines above the eastern horizon in predawn skies, completing its transition to planet Earth's morning star. On August 21, NASA's Parker Solar Probe completed its sixth gravity assist flyby of Venus, using the encounter to maneuver the probe toward its closest approach yet to the Sun.

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[revloren]

So which phase of Venus appears brightest from Earth? Is it brighter when it is bigger or when it is closer?

"Even though Venus exhibits the full suite of phases, its crescent phase near closest approach to Earth is when it’s at its brightest, but it’s only slightly fainter when it’s farthest away as it enters its full phase."

https://www.forbes.com/sites/startswith ... 0c5fc82d72

[George]

Re Venus’ apparent magnitude: According to the predicted light curve at https://theskylive.com/venus-info#ephemeris (tap green curve to read data for a particular date), Venus was brightest on 12 July 2023. On that date, Sky Safari says it was 22% illuminated, and about 36deg from the Sun. That was when the effect of thinning crescent exceeded the effect of increasing size.

[orin stepanek]

Do we ever get a full Venus view when on the far reaches of its Orbit?

[Joe Stieber]

Re Venus’ apparent magnitude: According to the predicted light curve at https://theskylive.com/venus-info#ephemeris (tap green curve to read data for a particular date), Venus was brightest on 12 July 2023. On that date, Sky Safari says it was 22% illuminated, and about 36deg from the Sun. That was when the effect of thinning crescent exceeded the effect of increasing size.

It sounds like you're describing the "Greatest Illuminated Extent," or GIE, which may be a more useful measure than "brightness." See this paper by Mark Gingrich in ALPO's Strolling Astronomer, Vol.42, No.1, January 2000, page 41...

https://alpo-astronomy.org/djalpo/42-1/ ... 1-free.PDF

Prior to inferior conjunction on August 13, 2023, Venus was at GIE on July 7, 2023, at 20 UT per the US Naval Observatory's booklet, Astronomical Phenomena 2023, page 6. The same date and time are echoed by the Royal Astronomical Society of Canada's Observer's Handbook 2023, page 111.

Using data from the USNO's MICA 2.2.2 software, I made a spreadsheet that calculated the illuminated area daily (at 20 UT) from Venus' diameter and fraction illuminated for the first half of July 2023. Indeed, the greatest illuminated area was on July 7, when Venus was 37.21" apparent diameter and 0.267 illuminated, for an area of about 290 arc-sec². WinJUPOS indicated a solar elongation of 39.2° at the time.

Venus' upcoming morning GIE is on September 19, 2023, at 07 UT.

[Joe Stieber]

Do we ever get a full Venus view when on the far reaches of its Orbit?

At superior conjunction, when Venus is on the far side of the Sun and typically not at much of an angular separation from the overwhelmingly brilliant solar disc, spotting the resultant "full" Venus can be difficult to say the least. I thought it would have been impossible until the Fall of 2022 when I was spotting Venus in the daytime leading up to superior conjunction on October 22. On that date, it would only be 1.05° from the Sun, center-to-center, not worth even trying to see, or so I thought.

Nevertheless, I was spotting Venus on every clear day leading up to superior conjunction, and we were lucky to have a good number of days with nice blue skies last October, and I just kept going to the day of conjunction. Using my 88 mm spotting scope and an approximate 4 x 6 ft solar panel about 15 ft up a local utility pole to BLOCK THE SUN, WHICH IS IMPERITIVE for both practical and safety reasons, I saw Venus mid-day on October 22, 2022, the day of superior conjunction. It was just a tiny bright disc that was difficult to pick out of the glare from the Sun that extended beyond the edge of the solar panel, not to mention the visual confusion from wind-borne seeds, which are surprisingly bright close to the Sun due to forward scattering -- but Venus was there!

Using the same 88 mm scope, I also saw Venus on the day of the prior inferior conjunction, January 8, 2022, and the following inferior conjunction on August 13, 2023, so that's three conjunctions in a row -- after many years of trying and failing to do so, usually being foiled by poor weather. Weather precluded an attempt at sighting the inferior conjunction on June 3, 2020, but I was able to see it on June 1, 2020, when the solar elongation was just 3.1°and Venus was a mere 0.1% illuminated. Of all my years of astronomical observing, that was one of the most spectacular views I've ever had. The delicately thin crescent, extending the full circumference in moments of better seeing, was beyond sublime. No photo of the crescent Venus I've ever seen gets close to that visual sighting.

[orin stepanek]

Do we ever get a full Venus view when on the far reaches of its Orbit?

At superior conjunction, when Venus is on the far side of the Sun and typically not at much of an angular separation from the overwhelmingly brilliant solar disc, spotting the resultant "full" Venus can be difficult to say the least. I thought it would have been impossible until the Fall of 2022 when I was spotting Venus in the daytime leading up to superior conjunction on October 22. On that date, it would only be 1.05° from the Sun, center-to-center, not worth even trying to see, or so I thought.

Nevertheless, I was spotting Venus on every clear day leading up to superior conjunction, and we were lucky to have a good number of days with nice blue skies last October, and I just kept going to the day of conjunction. Using my 88 mm spotting scope and an approximate 4 x 6 ft solar panel about 15 ft up a local utility pole to BLOCK THE SUN, WHICH IS IMPERITIVE for both practical and safety reasons, I saw Venus mid-day on October 22, 2022, the day of superior conjunction. It was just a tiny bright disc that was difficult to pick out of the glare from the Sun that extended beyond the edge of the solar panel, not to mention the visual confusion from wind-borne seeds, which are surprisingly bright close to the Sun due to forward scattering -- but Venus was there!

Using the same 88 mm scope, I also saw Venus on the day of the prior inferior conjunction, January 8, 2022, and the following inferior conjunction on August 13, 2023, so that's three conjunctions in a row -- after many years of trying and failing to do so, usually being foiled by poor weather. Weather precluded an attempt at sighting the inferior conjunction on June 3, 2020, but I was able to see it on June 1, 2020, when the solar elongation was just 3.1°and Venus was a mere 0.1% illuminated. Of all my years of astronomical observing, that was one of the most spectacular views I've ever had. The delicately thin crescent, extending the full circumference in moments of better seeing, was beyond sublime. No photo of the crescent Venus I've ever seen gets close to that visual sighting.

Thank you; I appreciate your response!

[George]

Re Venus’ apparent magnitude: According to the predicted light curve at https://theskylive.com/venus-info#ephemeris (tap green curve to read data for a particular date), Venus was brightest on 12 July 2023. On that date, Sky Safari says it was 22% illuminated, and about 36deg from the Sun. That was when the effect of thinning crescent exceeded the effect of increasing size.

It sounds like you're describing the "Greatest Illuminated Extent," or GIE, which may be a more useful measure than "brightness." See this paper by Mark Gingrich in ALPO's Strolling Astronomer, Vol.42, No.1, January 2000, page 41...

https://alpo-astronomy.org/djalpo/42-1/ ... 1-free.PDF

Prior to inferior conjunction on August 13, 2023, Venus was at GIE on July 7, 2023, at 20 UT per the US Naval Observatory's booklet, Astronomical Phenomena 2023, page 6. The same date and time are echoed by the Royal Astronomical Society of Canada's Observer's Handbook 2023, page 111.

Using data from the USNO's MICA 2.2.2 software, I made a spreadsheet that calculated the illuminated area daily (at 20 UT) from Venus' diameter and fraction illuminated for the first half of July 2023. Indeed, the greatest illuminated area was on July 7, when Venus was 37.21" apparent diameter and 0.267 illuminated, for an area of about 290 arc-sec². WinJUPOS indicated a solar elongation of 39.2° at the time.

Venus' upcoming morning GIE is on September 19, 2023, at 07 UT.

[George]

Oops! My guess as to how to quote *and reply to* a piece of a previous comment obviously failed! Sorry! Are there instructions somewhere?

What I *wanted* to say was that the link I included: A. says it was providing visual magnitudes (not Illuminated Extents); and B. (I later discovered) seems to provide data only at several-day intervals.

Also, does anyone know why the pre- and post-inferior conjunction peak visual magnitudes are different?