If you want to see Perseids - go to bed.

[Stub Mandrel]

I've been amazed by the results of radar detections of Perseid meteors. Although my counts include some false positives, such as ISS passes and spaec-junk, these are random, so averaging results over a week or so give a pretty good idea what times of day the meteors arrive:

On this graph '1' is between midnight UTC and 1:00am and so on. The peak is at 6:00am, which is 7:00 am  BST. Well after sunrise! In fact at dawn (about 4:00am UTC) the meteor count is only starting to rise rapidly. The hours leading up to midnight UTC (1:00 BST) are the quietest time for detections, rather later than most people assume.

My advice is to go to bed early and get up about 3:00am (BST) if you want the biggest chance of seeing Perseids - assuming the clouds allow.

 

[DRT]

Presumably this is good news for people on the east coast of the Americas?

[Dragon_Astro]

Interesting results there, it would explain why the "spectacular" show never really happens for those (like me) that go out around midnight as advised. We're 3 or 4 hours too early.

I suppose you'd have to repeat the experiment next year to see if this is a consistent time frame, or the peak time moves a bit.

[DRT]

10 minutes ago, Dragon_Astro said:

Interesting results there, it would explain why the "spectacular" show never really happens for those (like me) that go out around midnight as advised. We're 3 or 4 hours too early.

I suppose you'd have to repeat the experiment next year to see if this is a consistent time frame, or the peak time moves a bit.

Surely this is all to do with where in the Earth's rotation you happen to be at the point where the planet passes through the densest part of the meteor stream? (which would presumably vary year-on-year?)

[JamesF]

Random thought...

This shower is named the Perseids because they tend to originate from the area of Perseus.  At the time of greatest activity, Perseus is close to the zenith for the UK.  Could this be related to your findings?

James

[JamesF]

Come to think of it, Perseus is also close to due south at the same time.  If you're basing your measurements from a southerly radio source, might that also come into play?

James

[Stub Mandrel]

2 hours ago, DRT said:

Surely this is all to do with where in the Earth's rotation you happen to be at the point where the planet passes through the densest part of the meteor stream? (which would presumably vary year-on-year?)

No, it's a consistent peak every day for over a week. Might be affected by my aerial pointing south west, meaning the true peak is actaullly even later...

It may well vary for other showers that cross the earth's orbit at different speeds/angles.

1 hour ago, JamesF said:

Come to think of it, Perseus is also close to due south at the same time.  If you're basing your measurements from a southerly radio source, might that also come into play?

James

I don't thinks so. The general '6:00am peak' effect is well known, I just hadn't realised how marked the effect is.

[DRT]

Are you measuring a count of there number of meteorites hitting the atmosphere or some sort of intensity or duration of signal created by them?

If it is the latter, perhaps the peak occurs as the UK (or wherever) is turned sideways on to the stream of meteorites resulting in more long duration passes overhead with the late night lower reading being caused by us pointing straight at the stream with lots of them making a direct hit and burning up almost instantaneously?

[Stub Mandrel]

Number of hits, but sometimes long tracks can give multiple hits  - and also long tracks can mask smaller ones around the same times- it tends to balance out.

Also if that was an effect, there could be a peak at 6pm as well.

I've tried modelling it and aside from an obvious daily sinusiodal variation overlaid on a constant level, there is plenty more going on.

This is interesting: https://www.imo.net/observations/fireballs/rates/ it says that for fireballs the slower they hit, the deeper they penetrate and the brighter they are, so visual/photographic peak is at 6pm local time!

This: http://adsabs.harvard.edu/full/1969CoSka...4...46S is very complex and hard to understand but seems to indicate that visual and radar give different results and that the diurnal changes are not constant during the year - I suppose reflecting that meteor streams approach from different directions.

This plot from the BAA document "Detecting Meteors with Radar" shows the 'dawn spike' well:

 

[DRT]

My brain hurts! 

Assuming the meteors are not coming in waves with a period of 24 hrs this must all be down to the movement of the Earth in its own rotation and the procession of it's orbit around the Sun.

Looking at the above diagram it seems that the highest apparent velocity coincides with the spike in meteor hits. That could explain why were isn't a similar spike at 6pm?

However, my brain is telling me that diagram is wrong. Maximum apparent velocity should be at 0hrs where the rotation of the Earth and the orbit around the Sun are moving in the same direction. Why is it shown at dawn?

[Stub Mandrel]

16 minutes ago, DRT said:

Maximum apparent velocity should be at 0hrs where the rotation of the Earth and the orbit around the Sun are moving in the same direction. Why is it shown at dawn?

Even at the Equator the rotational speed is only 450m/s, less than 1/70 of the orbital speed so it only has a relatively small effect on apparent meteor speed, also it doesn't change where the meteors a hit.

[DRT]

1 minute ago, Stub Mandrel said:

No, the earth is travelling left to right around the sun in the diagram, so at dawn we are facing ahead, at noon we face the sun and are at 90 degrees to the direction of orbit and at 6pm we face backwards.

At dawn we are travelling left to right at 30km/s and bottom to top at right angles to that at 35km/s. That does not add up to 65km/s in any one direction.

At 0h we are travelling at 30km/s along the Earth's orbit and rotating at 35km/s in the same direction. That is an apparent velocity of 65km/s.

Think of the old maths exam question about a man running along a train. If he ran sideways from one side of the train to the other he would be travelling at the same sped as the train ? 

[Stub Mandrel]

8 minutes ago, DRT said:

At dawn we are travelling left to right at 30km/s and bottom to top at right angles to that at 35km/s. That does not add up to 65km/s in any one direction.

At 0h we are travelling at 30km/s along the Earth's orbit and rotating at 35km/s in the same direction. That is an apparent velocity of 65km/s.

Think of the old maths exam question about a man running along a train. If he ran sideways from one side of the train to the other he would be travelling at the same sped as the train ? 

I got muddled about your point and edited my post!

We are only rotating at 0.45km/s the 35 km/s is the speed of the meteors!

[DRT]

23 minutes ago, Stub Mandrel said:

I got muddled about your point and edited my post!

We are only rotating at 0.45km/s the 35 km/s is the speed of the meteors!

Ah! 

So what that diagram is really trying to convey is the closing velocity between the meteors and the Earth?

[DRT]

Sorry for all the questions, just trying to understand this as my A Level Physics and Maths happened more than three decades ago and have rarely been used since!

[DRT]

So in layman's language, the Earth hoovers-up meteorites as it and they pass through its orbital path around the Sun, and the mouth of the hoover is at whichever part of the Earth happens to be experiencing sunrise on any point in time.

I think I get it now.