# Earth and Moon from Cassini

## History of one investigation

In social networks – in groups and communities – there are regular publications that cause an ambiguous reaction. It’s normal for people to argue. This is much better than if they agreed with everything and took everything on faith – unconditionally and doomed. Moreover, it becomes possible to find that important missing link in our worldview, which does not allow many people to accept the very point of view that would be really constructive and correct.

The missing link is the void between the process of obtaining data and publishing it.

We no longer live at a time when one can refer authoritatively to authorities. It is necessary to prove, moreover, in the simplest and most understandable way.

## Let’s move on to the topic

Obviously, on the anniversary of the event, a photograph appeared on Facebook, taken by one of the cameras of the Cassini interplanetary station: View of the Earth and Moon from the vicinity of Saturn. In the picture there is only a radiant “asterisk” of the Earth, and a point of the Moon of average brightness. And another glare – the result of reflection in the lenses – from the Earth, presumably (but this is not accurate).

And of course, the question immediately followed:

### “Why can’t you see the stars?”

The question is not new. It has already been asked in a thousand disputes about “Did the Americans fly to the moon or not?”. But in the “Lunar theme” everything is clear – the brightness of the lunar landscape is the same as the brightness of the hot summer asphalt, blinding the eyes on a sunny day. What kind of stars are there, if the shutter speed when shooting is 1/1000 of a second – the stars need 1000 times more – at least.

But in deep space, when there is a distant Earth and Moon in the picture – they themselves are like stars, why are the stars not visible again?

## Let’s try to figure it out

The Earth-Moon system and Saturn are on average separated by a distance of about 10 astronomical units or one and a half billion kilometers. How bright are the Earth and Moon from this distance?

You can roughly estimate that the Earth is like Venus – the albedo (reflective power) of our planet is more than one and a half times weaker than the Venusian one. True, the Earth is slightly larger than Venus. Therefore, an estimate of “one and a half times” in terms of albedo will be just right. Venus from a distance of 1 astronomical unit is visible as a luminary minus the fourth magnitude. The Earth would be visible from such a distance one and a half times weaker – by half the magnitude – that is, minus the fourth magnitude would be the brightness of the Earth under the conditions in which we observe Venus – that is, from a distance of 1 astronomical unit.

Let’s move on to Saturn. The distance has increased 10 times. This means that the brightness of the Earth will decrease by 10 squared – a hundred times.

From an astronomy course (I hope to write a separate article on this topic), a 100-fold difference in brightness is exactly 5 magnitudes. We need to add one more, because the Earth is one and a half times farther from the Sun than Venus, which when squaring gives more than two times, and provides a brightness difference of 1 magnitude.

Therefore, the Earth from the orbit of Saturn will be six and a half magnitudes fainter than Venus in the sky of the Earth.

According to this very rough estimate, the brightness of the Earth in the image of the Cassini station is the second magnitude.

The Moon – in terms of albedo, it is three and a half times dimmer than the Earth, and the Moon is also three and a half times smaller in diameter (and the diameter ratio must be squared, because the apparent area of ​​\u200b\u200bthe object is critical for us). In total, the ratio of the brightness of the Earth and the Moon will be three and a half cubed, which is about 40 times, or 4 magnitudes.

The estimated stellar magnitude of the Moon, when observed from the vicinity of Saturn, will be only 6m – at the limit of visibility by the eye.

This means that if some stars were located near the line of sight towards the Earth and the Moon, accessible to the eye in terms of their brightness, they should have turned out in the picture.

But were there such bright stars in this region of the sky?

We have a way to test this.

### Program Stellarium

This is a virtual planetarium in your computer or smartphone that shows the view of the starry sky at a given date and time for some specific point on the globe. But Stellarium can show a view of the starry sky – with planets and small bodies of the solar system – for a virtual observer located on the surface of a celestial body. We can travel to one of Saturn’s moons and look towards the Earth through the eyes of Cassini.

Saturn’s moon Mimas is another “Death Star”

## Which moon of Saturn to choose?

I propose Tethys – a kind of “Death Star” from Star Wars – the owner of the largest impact crater in the solar system. To do this, you just need to select the name of the satellite in the drop-down list of celestial bodies, to which the Stellarium program provides direct and regular flights. we can even choose a landing point – for example, the bottom of that very huge crater called “Odysseus”.

And we also need to travel back to the date when the photograph in question was taken – July 19, 2013.

The flight in space and time is almost instantaneous. A fascinating picture appears before our eyes:

We look at Saturn from the unlit rear of the Sun. In addition to the stars and constellations known to us, a certain number of luminaries unusual for an earthling shine in the sky – this is a retinue of satellites of the “Lord of Time”, which was considered Kronos by the ancient Greeks, familiar to us by his Roman name – Saturn. But not only the satellites of Saturn decorate the sky of the outskirts of the solar system. Several planets can be found around the distant Sun. Somewhere among them lies the Earth with the Moon. You can’t tell right away where the Earth is.

Fortunately, celestial objects are signed in Stellarium. I deliberately turned off the hints in order to feel the unfamiliar view of the Saturnian skies. But if you turn the markers back on, everything becomes clear:

The earth was very close to the sun. And this is not surprising, because the orbit of the Earth is deep inside the orbit of Saturn. And for an observer of these cold and distant edges, the Earth does not move further from the Sun than 7 degrees. This is very little. The elusive Mercury from the Earth is difficult to observe precisely because of its proximity to the Sun, and in fact it moves away from the Sun by 27 degrees at its maximum elongations. And still, few people saw him. This I mean that photographing the Earth from the vicinity of Saturn is very difficult because of the apparent proximity to the Sun.

On this day – July 19, 2013 – the elongation of the Earth was just at its maximum. We were lucky. And most likely this time for shooting was not chosen by chance.

## Well, where is the moon?

To do this, we need to significantly increase the scale – apply literally telescopic zoom.

In the process of zooming in, numerous stars will appear on the screen and scatter to its edges, but the Moon will be visible separately from the Earth only when the faintest stars that Stellarium shows disappear from our field of view.

This does not mean that there are no stars in this direction in the Universe. Simply, they are so weak that even Stellarium knows nothing about them.

However, if you zoom out back and find the faintest star closest to the Earth and the Moon in the sky, you can inquire about it – it will be weaker than the 10th magnitude. But even she would have remained outside the field of view of the camera of the interplanetary station Cassini.

From the Saturnian distances, the Earth and the Moon practically merge. The distance between them on this day was only 24 arc seconds – this is one and a half times less than between the components of the double star Albireo. When photographing Albireo, most often a frame is also obtained in which there are only two components of the system, and – that’s all – almost no other stars are visible.

We can zoom in even further and see the Earth and Moon in more detail. It’s educational.

By the way, Stellarium easily calculates the brightness of both luminaries. According to his data, the brightness of the Earth is 2.85m, and the brightness of the Moon is 7.16m. In my proposed quick and rough estimates, we were wrong on average by 1 magnitude. And as you can understand, the camera of the Cassini station takes medium-brightness stars confidently, it’s just that they were simply not in the field of view with a diameter of several arc minutes (thanks to Stellarium – he tells you that too). But, of course, there are some stars there that are simply inaccessible to cameras … or …

### What if we eat them?

I downloaded the picture from Facebook and turned up the brightness in the image editor as much as was reasonable.

And something resembling stars came out in the picture. Although, there is no guarantee that these are stars – we cannot identify them with real luminaries – Stellarium is not a helper here. Perhaps these are just compression artifacts, because the picture was downloaded from the social network, and there it was compressed many times and ruthlessly.

But I was not too lazy to look for something alternative. After all, this picture exists not only on Facebook. And I found a similar image on the Phys.org website – in a publication describing exactly the topic under discussion. And the picture quality is the best. And on it – here, a surprise! – there are stars.

## What are the comments here?

The pictures are in different orientations. perhaps these are just the central fragments of some large and unsuitable for publication source code, cut out in different ways for certain articles and reports. Or maybe these are pictures taken at different times and from different positions of the space station with different field orientations. Because I tried to match the suspicious stellar objects in both shots, and they didn’t match for the most part, and it would be weird if Cassini only took one shot of each target he chose. That doesn’t happen. Or maybe there really are no stars on the Facebook image – only noise and compression artifacts. But, at least, we were convinced that this is a completely plausible situation – getting into the frame with the Earth and the moon of some more or less bright star is rather a lot of luck than an obligatory matter.