APOD: Venus and the Triply Ultraviolet Sun (2018 Feb 04)

[APOD Robot]

Venus and the Triply Ultraviolet Sun

Explanation: An unusual type of solar eclipse occurred in 2012. Usually it is the Earth's Moon that eclipses the Sun. That year, most unusually, the planet Venus took a turn. Like a solar eclipse by the Moon, the phase of Venus became a continually thinner crescent as Venus became increasingly better aligned with the Sun. Eventually the alignment became perfect and the phase of Venus dropped to zero. The dark spot of Venus crossed our parent star. The situation could technically be labeled a Venusian annular eclipse with an extraordinarily large ring of fire. Pictured here during the occultation, the Sun was imaged in three colors of ultraviolet light by the Earth-orbiting Solar Dynamics Observatory, with the dark region toward the right corresponding to a coronal hole. Hours later, as Venus continued in its orbit, a slight crescent phase appeared again. The next Venusian transit across the Sun will occur in 2117.

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[JohnD]

"Eclipse" seems a strange way to describe a transit of Venus, when the word also means "deprivation of light". We can detect the planets of other stars by seeing this, but only just, and would never notice on Earth if Venus were in front of the Sun or not.

More interesting is that transits, of Mercury as well as Venus, occur in pairs, a few years apart, and then only again many years later. See: https://eclipse.gsfc.nasa.gov/transit/c ... talog.html

And even more important, it was scientific theory, the development of the solar-centric model and the theory of gravity that allowed Edmund Halley to suggest that a transit would allow the distance from the Earth to the Sun to calculated. See; http://www.exploratorium.edu/venus/question4.html

And, that to enable that calculation, observation from distant parts of the Earth were necessary, and one of the objectives of James Cook's first circumnavigation of the Earth was to see the transit of Venus from Tahiti, in 1769.

So the Transit of Venus is an exciting and important part of the development of physics, astronomy, geography and exploration! All quite as exciting as that dramatic image of the sun in ultraviolet!
JOhn

[De58te]

I remember watching it right in front of my house back in 2004. Then they called it a Transit of Venus instead of a Venus Solar Eclipse. I was lucky because in the Toronto area it happened just a few minutes after sunrise, when I could look directly at the Sun with my own eyes. I could see Venus with my own eyes. It was a black dot on the lower part of the Sun instead of the upper like in the photograph. Lower part being the part closest to the horizon during the sunrise.

[BDanielMayfield]

A quite beautiful APOD. I also like John's take:

"Eclipse" seems a strange way to describe a transit of Venus, when the word also means "deprivation of light". We can detect the planets of other stars by seeing this, but only just, and would never notice on Earth if Venus were in front of the Sun or not.

More interesting is that transits, of Mercury as well as Venus, occur in pairs, a few years apart, and then only again many years later. See: https://eclipse.gsfc.nasa.gov/transit/c ... talog.html

And even more important, it was scientific theory, the development of the solar-centric model and the theory of gravity that allowed Edmund Halley to suggest that a transit would allow the distance from the Earth to the Sun to [be] calculated. See; http://www.exploratorium.edu/venus/question4.html

And, that to enable that calculation, observation from distant parts of the Earth were necessary, and one of the objectives of James Cook's first circumnavigation of the Earth was to see the transit of Venus from Tahiti, in 1769.

So the Transit of Venus is an exciting and important part of the development of physics, astronomy, geography and exploration! All quite as exciting as that dramatic image of the sun in ultraviolet!
JOhn

From the link John provided:

How do we know how far away the Sun is? Now we use radar to establish our distance from celestial objects, but before there was radar, we had to resort to trigonometry to figure out the astronomical unit (AU)—the distance between Earth and the Sun. In 1677, English astronomer Edmond Halley (of comet fame) proposed that a transit of Venus—and some geometry—could be used to determine the astronomical unit. Though Halley died in 1742, which was 19 years before the 1761 transit of Venus, other scientists followed his suggestion and journeyed to the ends of the Earth to make the necessary observations. The astronomical unit obtained from these observations, roughly 95 million miles, is respectably close to our current measure of 92,955,807.267 miles (149,597,870.691 kilometers).

I remember reading about Halley's call to action that he made back in 1677 re the 1761 transit of Venus. I found it a little amusing that he said expeditions should be planed "with all possible dispatch!" so long before the event.

Bruce

[Chris Peterson]

"Eclipse" seems a strange way to describe a transit of Venus, when the word also means "deprivation of light". We can detect the planets of other stars by seeing this, but only just, and would never notice on Earth if Venus were in front of the Sun or not.

With our eyes, that's true. But our instruments which monitor total solar flux easily detect the decrease in intensity of the sunlight during a Venus transit.

Click to view full size image

[Ann]

In this picture, Venus is the size of a rather big and dark sunspot against the disk of the Sun. I guess that when astronomers monitor other stars to determine if they have planets, a sunspot on another sun might fool astronomers into thinking that they have detected an exoplanet.

Oh well. I guess follow-up observations would show that the sunspot was not a planet.

Ann

[Chris Peterson]

In this picture, Venus is the size of a rather big and dark sunspot against the disk of the Sun. I guess that when astronomers monitor other stars to determine if they have planets, a sunspot on another sun might fool astronomers into thinking that they have detected an exoplanet.

Oh well. I guess follow-up observations would show that the sunspot was not a planet.

Yup. They're actually called starspots. And they produce variability that is equivalent to planetary transits. So it normally requires detecting a minimum of three transits to provisionally confirm you've detected a planet.

Starspots themselves can provide useful information about the rotation of other stars.

[Bellerophon]

I don't think the public really has much of any idea of the sun's size compared to the earth, and a neat feature of this photo is that it gives some idea of the relative sizes of the sun and Venus, and therefore of the sun and the earth.

Of course, Venus is only a little over a quarter to the way to the sun from the earth in this picture, so the viewer needs to imagine its disc shrunk to a little more than one quarter of what it is in this picture.

[neufer]

[size=150]Mercury Transit ~ 62% of Earth's size[/size]

---

I don't think the public really has much of any idea of the sun's size compared to the earth, and a neat feature of this photo is that it gives some idea of the relative sizes of the sun and Venus, and therefore of the sun and the earth.

Of course, Venus is only a little over a quarter to the way to the sun from the earth in this picture, so the viewer needs to imagine its disc shrunk to a little more than one quarter of what it is in this picture.

Indeed...

a Mercury Transit is a somewhat better

• representation of the Sun/Earth relative size.

[neufer]

In this picture, Venus is the size of a rather big and dark sunspot against the disk of the Sun. I guess that when astronomers monitor other stars to determine if they have planets, a sunspot on another sun might fool astronomers into thinking that they have detected an exoplanet.

Oh well. I guess follow-up observations would show that the sunspot was not a planet.

Yup. They're actually called starspots. And they produce variability that is equivalent to planetary transits. So it normally requires detecting a minimum of three transits to provisionally confirm you've detected a planet.

If the transit takes a whole day or more it is probably a starspot.

If it is a starspot that returns then the return time should be exactly twice the transit time.

[Chris Peterson]

In this picture, Venus is the size of a rather big and dark sunspot against the disk of the Sun. I guess that when astronomers monitor other stars to determine if they have planets, a sunspot on another sun might fool astronomers into thinking that they have detected an exoplanet.

Oh well. I guess follow-up observations would show that the sunspot was not a planet.

Yup. They're actually called starspots. And they produce variability that is equivalent to planetary transits. So it normally requires detecting a minimum of three transits to provisionally confirm you've detected a planet.

If the transit takes a whole day or more it is probably a starspot.

If it is a starspot that returns then the return time should be exactly twice the transit time.

Yeah, but often the starspots aren't that stable, so when they return they're a different size and not obviously the same ones. And on stars with fast rotation and a lot of activity, there tend to be a lot of starspots, which further confuses things.

[Leon1949Green]

So, when will the next Earthian transit of the Sun be seen from Mars?

[neufer]

So, when will the next Earthian transit of the Sun be seen from Mars?

---

<<No one has ever seen a transit of Earth from Mars, but the next transit will take place on November 10, 2084. The last such transit took place on May 11, 1984. A science fiction short story published in 1971 by Arthur C. Clarke, called "Transit of Earth", depicts a doomed astronaut on Mars observing the transit in 1984. During the event, the Moon could almost always also be seen in transit, although due to the distance between Earth and Moon, sometimes one body completes the transit before the other begins (this last occurred in the 1800 transit, and will happen again in 2394).

A transit of Earth from Mars corresponds to Mars being perfectly uniformly illuminated at opposition from Earth, its phase being 180.0° without any defect of illumination. During the 1879 event, this permitted Charles Augustus Young to attempt a careful measurement of the oblateness (polar compression) of Mars. He obtained the value 1/219, or 0.0046. This is close to the modern value of 1/154. Much more recently, better measurements of the oblateness of Mars have been made by using radar from the Earth. Also, better measurements have been made by using artificial satellites that have been put into orbit around Mars, including Mariner 9, Viking 1, Viking 2, and Soviet orbiters, and the more recent orbiters that have been sent from the Earth to Mars.

Transits of Earth from Mars usually occur in pairs, with one following the other after 79 years; rarely, there are three in the series. The transits also follow a 284-year cycle, occurring at intervals of 100.5, 79, 25.5, and 79 years; a transit falling on a particular date is usually followed by another transit 284 years later. Transits occurring when Mars is at its ascending node are in May, those at descending node happen in November. This cycle corresponds fairly closely to 151 Mars orbits, 284 Earth orbits, and 133 synodic periods, and is analogous to the cycle of transits of Venus from Earth, which follow a cycle of 243 years (121.5, 8, 105.5, 8). There are currently four such active series, containing from 8 to 25 transits. A new one is set to begin in 2394. The last series ending was in 1211.>>

[JohnD]

"Eclipse" seems a strange way to describe a transit of Venus, when the word also means "deprivation of light". We can detect the planets of other stars by seeing this, but only just, and would never notice on Earth if Venus were in front of the Sun or not.

With our eyes, that's true. But our instruments which monitor total solar flux easily detect the decrease in intensity of the sunlight during a Venus transit.

Click to view full size image

Well, yes, Chris, with your super-duper instruments that are modern astronomy. And they can detect a whole 1.5 watts/m^2 change in the Sun's radiance as Venus crosses it's disc, that's 0.11%. Remarkable, extraordinary, laudable. But I still stand by my statement that we, humans (not astronomers!) cannot tell if Venus is there or not, and compared with the same situation with the Moon it's not an eclipse!

John

[Chris Peterson]

Well, yes, Chris, with your super-duper instruments that are modern astronomy. And they can detect a whole 1.5 watts/m^2 change in the Sun's radiance as Venus crosses it's disc, that's 0.11%. Remarkable, extraordinary, laudable. But I still stand by my statement that we, humans (not astronomers!) cannot tell if Venus is there or not, and compared with the same situation with the Moon it's not an eclipse!

Well, sure. I said as much. I was just addressing the comparison between a Venus transit from Earth and the sort of transits we are able to detect by planets around other stars.

Technically, it isn't incorrect to call such events eclipses. "Eclipse" is a blanket term that covers any sort of obscuration, both partial (transit) or total (occultation). But certainly, "transit" is more precise.

FWIW, it is possible to see a Venus transit with the naked eye just like we see sunspots, typically with the Sun attenuated by smoke, clouds, or near sunset/sunrise. There's no solid evidence anybody did so before modern times, but there are weak indications. Of course, many astronomical things were probably observed in ancient times and never recorded.

[neufer]

<<An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer. This alignment of three celestial objects is known as a syzygy. Apart from syzygy, the term eclipse is also used when a spacecraft reaches a position where it can observe two celestial bodies so aligned. An eclipse is the result of either an occultation (completely hidden) or a transit (partially hidden). During a solar eclipse, the Moon can sometimes perfectly cover the Sun because its apparent size is nearly the same as the Sun's when viewed from the Earth. A total solar eclipse is in fact an occultation while an annular solar eclipse is a transit.

The term is derived from the ancient Greek noun ἔκλειψις (ékleipsis), which means "the abandonment", "the downfall", or "the darkening of a heavenly body", which is derived from the verb ἐκλείπω (ekleípō) which means "to abandon", "to darken", or "to cease to exist," a combination of prefix ἐκ- (ek-), from preposition ἐκ (ek), "out," and of verb λείπω (leípō), "to be absent".>>

[JohnD]

Chris notes that a transit at sunset (or sunrise) with sunlight attenuated by the greater depth of atmosphere, is observable, naked-eye, and De58te wrote above of doing just that.
But should this responsible site not record and support an absolute prohibition of looking at the Sun without protection for the eye?

John

[Chris Peterson]

Chris notes that a transit at sunset (or sunrise) with sunlight attenuated by the greater depth of atmosphere, is observable, naked-eye, and De58te wrote above of doing just that.
But should this responsible site not record and support an absolute prohibition of looking at the Sun without protection for the eye?

I suppose the editors can opt for whatever advice they like in their captions and their comments. I would never make such a claim. It is not dangerous to look (without staring) at a Sun attenuated by the atmosphere to the point where the blink or aversion response is no longer present. My advice would be to exercise care, but don't deny yourself the opportunity to see first hand one of nature's interesting sights (e.g. sunspots on a setting Sun).