The 2nd brightest object in the night sky?

[Oafman]

So I watched the ISS cross the night sky the other evening, for the first time. It's crazy that it took me so long to get around to doing this, and once I saw how obvious it is, I couldn't believe I had never 'accidentally' seen it. But I do live in England, we usually just see clouds.

Anyway, I had read that it is the 2nd brightest object, and while it was definitely bright, it did appear to the somewhat less bright than Venus. Obviously they weren't next to each other, so it wasn't too easy to make a comparison, but we all agreed that Venus looked brighter.

Venus was fairly close to the horizon, while the ISS passed overhead at 80', so I suppose we were viewing Venus through more atmosphere. But wouldn't that reduce Venus' brightness?

Can anyone explain why Venus appeared to be brighter? This was around midnight, so perhaps slightly over 2hours after sunset, if that is relevant.

Another thing it got me wondering was, how do we empirically determine which is brighter? I read a little about magnitude of objects in space but it wasn't clear how such things are calculated.

Thanks!

[ZetaX]

To determine which is brighter, you measure the amount of light from these objects that hit a given area (e.g. your eye). The apparent magnitude is somewhat stupid for introducing several random variables (0 is vega under good conditions and the scale is logarithmic with a totally random factor of 100^(1/5)), but you could calculate the magnitude from the light received (take the logarithm for that base and add an appropriate constant).

But why would you expect the ISS to be brighter than venus¿ Brightness depends on size, distance and albedo (i.e. how well it reflects). Venus obviously 'wins' by a lot in regard to size, looses by a lot for distance and should have a slightly worse albedo (not sure on the latter, but number's surely exist on the internet). But in the end, it could be easily possible that the distance of venus is not far enough away to cancel out it's 'advantage' in size.

[AngelLestat]

So I watched the ISS cross the night sky the other evening, for the first time. It's crazy that it took me so long to get around to doing this, and once I saw how obvious it is, I couldn't believe I had never 'accidentally' seen it. But I do live in England, we usually just see clouds.

Why you mean by accidentally? You can only see it for 3 or 6 min every 2 weeks or something like that.

This is because the time of the day, the sun and the spacestation needs to be in the right spot according to your place.

Venus was fairly close to the horizon, while the ISS passed overhead at 80', so I suppose we were viewing Venus through more atmosphere. But wouldn't that reduce Venus' brightness?

Atmosphere magnify objects, but it does not increase the brightness I guess, also being so small does not decrease it either.. But better if someone with real knowledge about astronomy answer that.

[tater]

The atmosphere doesn't magnify to any meaningful extent. Some scattering. The Moon looks bigger because it is near references.

Venus is large, relatively close to the earth, and relatively close to the Sun. Venus is currently between us and the sun, but off to the side. As a result it is pretty bight, and fairly displaced from the sun (a "full venus would be behind the sun, and nearly full venus is very close to the sun at sunset/sunrise.

[Oafman]

To determine which is brighter, you measure the amount of light from these objects that hit a given area (e.g. your eye). The apparent magnitude is somewhat stupid for introducing several random variables (0 is vega under good conditions and the scale is logarithmic with a totally random factor of 100^(1/5)), but you could calculate the magnitude from the light received (take the logarithm for that base and add an appropriate constant).

Thanks, that's interesting. But how do we actually measure it? With what device?

But why would you expect the ISS to be brighter than venus¿ Brightness depends on size, distance and albedo (i.e. how well it reflects). Venus obviously 'wins' by a lot in regard to size, looses by a lot for distance and should have a slightly worse albedo (not sure on the latter, but number's surely exist on the internet). But in the end, it could be easily possible that the distance of venus is not far enough away to cancel out it's 'advantage' in size.

Well I wouldn't have expected the ISS to be brighter, given that Venus is a large planet with a dense atmosphere. It's just that I had read somewhere that the ISS is brighter.

Would the timing of the viewing affect the ISS's brightness in some way? Would it have appeared brighter had I seen it 90 mins earlier, or if it had been right above me at 90'?

[AngelLestat]

ah ok tater. thanks for clarification.. that seems logic.

About venus albedo.. this is big due its sulfure clouds which reflect sunlight better than water clouds.

Not sure how much light solar panels reflect, in theory they need to be designed to absorb light..

[Oafman]

Why you mean by accidentally? You can only see it for 3 or 6 min every 2 weeks or something like that.

This is because the time of the day, the sun and the spacestation needs to be in the right spot according to your place.

It's visible more often than that. When I looked up possible flyovers, there were 2 or 3 every evening, for a few days in a row. But I do understand that I need my garden to be underneath the high latitude point of the ISS's orbit, so I suppose it's just a few days of possible viewings in between periods of a couple of weeks when it can't be seen from here.

And by 'accidentally' I just meant seeing it when I wasn't trying to see it.

- - - Updated - - -

The atmosphere doesn't magnify to any meaningful extent. Some scattering. The Moon looks bigger because it is near references.

Venus is large, relatively close to the earth, and relatively close to the Sun. Venus is currently between us and the sun, but off to the side. As a result it is pretty bight, and fairly displaced from the sun (a "full venus would be behind the sun, and nearly full venus is very close to the sun at sunset/sunrise.

So, when the claim is made that the ISS is brighter than Venus, it just applies to times when Venus is less bright than it currently is?

[tater]

ISS has some large, flat reflectors (the solar panels). Since the attitude of ISS is known, on certain flyovers, from certain points of view on earth, the sun is reflected directly at the viewer. Think about a signal mirror. At some random angle to you, it's not bright at all. If I angle it so that the sun is reflected into your eye, it's blinding.

There is a phone app called "Iridium Flares" that shows such passes by the Iridium sat phone satellites. These are pretty small, and they can be substantially brighter than Venus (out to ~ -8 magnitude (lower is brighter, and it's a log scale). (cool app, it's like a meteor on command" thing. Check when a flare will occur, then wait til a few seconds before and tell your friends to look the right direction---a dot appears, gets crazy bright, then dims back down to a dot).

Venus is doing the same thing, reflecting sunlight, but it is much farther away than ISS. The mirror a few meters from you is far closer than either, so it would be brighter than anything but the sun.

Apparent magnitude is what we see at any given moment. Absolute magnitude is how bright objects appear if they are all seen from exactly the same distance (set to 10 parsecs). At 10 parsecs, ISS is invisibly dim.

[Oafman]

So technically, it's only the 'flare' from ISS which will only be visible on a specific and small part of the Earth's surface, that is brighter than Venus?

I want that app. Is it just on iPhone? I can see a few for android: eg Iridium and Iridium Flare Forecast, but no 'Iridium Flares'

[UmbralRaptor]

Thanks, that's interesting. But how do we actually measure it? With what device?

Typically specialized cameras. You essentially draw a circle around the source and add up all the pixel values inside to get brightness. Converting this into a flux may require some familiarity with the hardware through previous testing and/or having something of known brightness in the same image.

So, when the claim is made that the ISS is brighter than Venus, it just applies to times when Venus is less bright than it currently is?

I couldn't find any passes (in a cursory search) as bright as -4 magnitude (Venus gets up to about -4.5), so probably. Also, I'd expect Iridium flares to peak at brighter than ISS passes.

[PakledHostage]

It's visible more often than that. When I looked up possible flyovers, there were 2 or 3 every evening, for a few days in a row. But I do understand that I need my garden to be underneath the high latitude point of the ISS's orbit, so I suppose it's just a few days of possible viewings in between periods of a couple of weeks when it can't be seen from here.

You can see it more readily in the summer than in the winter from your location in the UK because it is illuminated by sunlight for more passes and for longer periods in summer than it is in winter.

I just checked and the Sun only gets down to roughly 17 degrees below London's horizon this evening. The ISS orbits at roughly 400 km which is sufficient for it to still be in sunlight when the Sun is as much as about 20 degrees below the horizon. In other words, if the ISS was directly overhead London at 1:00 am (daylight savings time) tonight, the ISS would still be in sunlight while it was celestial midnight on the ground. That sunlight would obviously make it visible to viewers on the ground.

In winter, you only see it in the western part of the sky before it crosses the terminator. When it does, it kind of fades out. Somewhere I have a long exposure picture of it fading out as it crosses the terminator, but I'd have to search for it.

I have seen it "accidentally" while out in the evening. You can infer that it must be the ISS because it is so bright and doesn't "flare" like an iridium satellite. In summer in northern latitudes, you can also wait for it to come back around and watch it go by on the next pass about 90 minutes later.

Edited June 5, 2015 by PakledHostage
clarified a point.

[lajoswinkler]

So I watched the ISS cross the night sky the other evening, for the first time. It's crazy that it took me so long to get around to doing this, and once I saw how obvious it is, I couldn't believe I had never 'accidentally' seen it. But I do live in England, we usually just see clouds.

Anyway, I had read that it is the 2nd brightest object, and while it was definitely bright, it did appear to the somewhat less bright than Venus. Obviously they weren't next to each other, so it wasn't too easy to make a comparison, but we all agreed that Venus looked brighter.

Venus was fairly close to the horizon, while the ISS passed overhead at 80', so I suppose we were viewing Venus through more atmosphere. But wouldn't that reduce Venus' brightness?

Can anyone explain why Venus appeared to be brighter? This was around midnight, so perhaps slightly over 2hours after sunset, if that is relevant.

Another thing it got me wondering was, how do we empirically determine which is brighter? I read a little about magnitude of objects in space but it wasn't clear how such things are calculated.

Thanks!

Ignoring the misty/dusty sky near horizon, Venus has a fairly low oscillation of magnitude and it's slow. It's a question of months.

ISS varies greatly in magnitude. It can reach just over -3.3 m. I see Wikipedia mentions -6. I suppose those are sudden flashes of its solar panels, similar to Iridium flashes. It will never be that bright on its own.

If Venus is really low on a dusty horizon, and ISS is passing around zenith, the station might very well be apparently brighter.

To accurately determine the apparent magnitude, you need a device.

[AngelLestat]

It's visible more often than that. When I looked up possible flyovers, there were 2 or 3 every evening, for a few days in a row. But I do understand that I need my garden to be underneath the high latitude point of the ISS's orbit, so I suppose it's just a few days of possible viewings in between periods of a couple of weeks when it can't be seen from here.

And by 'accidentally' I just meant seeing it when I wasn't trying to see it.

Yeah, 2 days in a row every 2 or 3 weeks only for 3 or 5 mins looking in the right direction. Not sure if the conditions are different for a diferent place.

http://spotthestation.nasa.gov/sightings/

You have to be very lucky to see it, if you random look up the sky.

Edited June 5, 2015 by AngelLestat

[Mr Shifty]

The atmosphere doesn't magnify to any meaningful extent. Some scattering. The Moon looks bigger because it is near references.

Venus is large, relatively close to the earth, and relatively close to the Sun. Venus is currently between us and the sun, but off to the side. As a result it is pretty bight, and fairly displaced from the sun (a "full venus would be behind the sun, and nearly full venus is very close to the sun at sunset/sunrise.

We can't ever see a full Venus from Earth, as it would only be visible during the day. Inferior planets are only ever visible near sunrise or sunset and only in half-phase or less. Tomorrow is actually Venus's greatest western elongation, so it's as bright as it will get for another few months as it crosses 'in front' of the sun and shows its dark side to us.

[tater]

We can't ever see a full Venus from Earth, as it would only be visible during the day. Inferior planets are only ever visible near sunrise or sunset and only in half-phase or less. Tomorrow is actually Venus's greatest western elongation, so it's as bright as it will get for another few months as it crosses 'in front' of the sun and shows its dark side to us.

I said if full it would be behind the sun. I assumed that any reader would recognize that people (or instruments) cannot see through the sun. Silly me.

The tone of OP's questions drove the level at which I chose to answer, telling him it was near greatest elongation would have required an explanation of that as well.

btw, Venus is absolutely visible on the other side of the sun (meaning well past half phase). It might not be easy to see naked eye, but you can see it (it is considerably smaller due to distance however).

Edited June 5, 2015 by tater

[ZetaX]

We can't ever see a full Venus from Earth, as it would only be visible during the day.

If you go as nitpicky as that: the actual reason is that the sun would be between Venus and Earth.

Inferior planets are only ever visible [...] in half-phase or less.

Not true. 50% happens at the maximal elongations, yes, but it goes over 50% beyond that. That doesn't mean it's brighter (because it will be further away) or better to observe (because it will appear closer to the sun).

[Oafman]

Well, I saw 2 Iridium flares and 3 ISS flyovers last night. This is easy!

One flyover precisely coincided with an Iridium flare, and they were only about 5 degrees apart. It would have made a great long exposure photo.

The Heavens Above app is very good

The tone of OP's questions drove the level at which I chose to answer, telling him it was near greatest elongation would have required an explanation of that as well.

Actually I would prefer correct terms, I can google whatever I don't recognise

Edited June 6, 2015 by Oafman

[maltesh]

Yeah, 2 days in a row every 2 or 3 weeks only for 3 or 5 mins looking in the right direction. Not sure if the conditions are different for a diferent place.

http://spotthestation.nasa.gov/sightings/

You have to be very lucky to see it, if you random look up the sky.

One of the things I wound up doing several years ago, was setting up something that would scrape Heavens Above for Satellite Posses over my location, and post them to my Google Calendar. As a result I'd say that days in which the ISS /is/ visible in the morning or evening are more common than days in which they aren't. I usually have several weeks of daily visible passes, followed by a week or two where it's not visible as it switches from Evening to morning, or vice versa..

The calendar pulls the next 10 days' worth of ISS sightings and Iridium Flares off Heavens Above, (and for some reason, seems to have not updated since Wednesday. I should look into that.)This is kind of typical for mid June, where there are nights with three or four sightings. Most of the year, it's either one or two visible passes per day in the evening or morning.

That said, I live at about 41° N latitude, and quick futzing around with Heavens above seems to show visible passes seem a /lot/ rarer at the equator.

[Oafman]

The calendar pulls the next 10 days' worth of ISS sightings and Iridium Flares off Heavens Above.

Not only does it warn you what's coming, but if you're at work, and someone sees it over your shoulder, it makes you look really busy.

[Findthepin1]

I'm pretty sure it's Sirius.

[Oafman]

I'm pretty sure it's Sirius.

You sound like an ISS hoaxer!

I recently learned that such people do exist. I wondered how they might explain the fact that it is visible to the naked eye. So I googled it, and apparently it's a really big balloon. Which moves really quickly.

[Findthepin1]

The ISS exists, but it is only over a certain relatively small area of Earth at a time and only for a short duration over that area, and it is to be decommissioned in 2020. Sirius is viewable from a large fraction of the planet at any given time.

[Oafman]

Oh right, sorry, you meant that Sirius is the 2nd brightest object? I think that would only be true if no planets were visible

[lajoswinkler]

Sirius is the brightest star in the sky and we (people living on northern hemisphere) can't see it now. Venus is much brighter than Sirius.

[Oafman]

You can see it more readily in the summer than in the winter from your location in the UK because it is illuminated by sunlight for more passes and for longer periods in summer than it is in winter.

I just checked and the Sun only gets down to roughly 17 degrees below London's horizon this evening. The ISS orbits at roughly 400 km which is sufficient for it to still be in sunlight when the Sun is as much as about 20 degrees below the horizon. In other words, if the ISS was directly overhead London at 1:00 am (daylight savings time) tonight, the ISS would still be in sunlight while it was celestial midnight on the ground. That sunlight would obviously make it visible to viewers on the ground.

In winter, you only see it in the western part of the sky before it crosses the terminator. When it does, it kind of fades out. Somewhere I have a long exposure picture of it fading out as it crosses the terminator, but I'd have to search for it.

I had to think about this for a while before I fully understood what causes the differences in flyovers between northern hemisphere summers and winters. If anyone else is not clear these diagrams helped: