Shadows

[bongerman]

I cycle to work.

I've noticed in the last couple of weeks that as I cycle along my road in the morning my shadow is EXACTLY in front of me (ie sun is directly behind me)

Last night as I cycled home (the other way on the SAME bit of road) I noticed this phenomenon to be true once more. I was cycling directly onto my shadow with the sun directly behind me.

Is this sufficient information to tell how many hours/minutes there are between these two times?

Will this phenomenon continue - clearly it can't continue all year as in the winter I cycle both legs in the dark!

Any answers/comments appreciated.

Thanks in advance.

[gen2]

12 hours

The earth rotates 360 degrees in 24 hours so will turn 180 degrees in 12 hours.

As you realise, this cannot continue once the day length becomes less than 12 hours long.

[gen2]

You are acting like a sundial. If you refer to exactly the same stretch of rod, then the sun will not be directly behind you at exactly the same time every day, but will occur up to 15 minutes earlier or later depending on the time of year. You can find out how many minutes the sun is ahead or behind clock time by looking up 'The Equation of Time' for the desired date.

[stevie21]

If the sun moves from being directly in front of you, to overhead, to directly behind you then at any time at all either side of it being overhead your 2 shadows will form a 180 degree angle.

That's *if* it passes directly overhead though

[brachiopod]

No.

Unless, you knew your exact latitude, longitude and the date.

If you were on the equator, and cycled to and from work on a road running exactly East - West, then, on (or about) the 21st March and the 21st September, your shadow (assuming travelling westwards to work) would be exactly in front of you, and, on your return journey (Eastwards) it would be exactly behind you.

However, you could wake up with a hangover and not get to work until 11.30 (local time) and go home an hour later at 12.30 (local time) and the same would apply - your shadow would be in-front / behind you just as if you had done a full day.

Your bike is not tall enough to throw an accurately measurable shadow - the errors associated with measuring the shadow's angle are to great to calculate an accurate time - even if you knew the time of year (date), latitude and longitude.

[bongerman]

Thanks for your replies.

gen2 - it's not 12 hours. It's 7.20 a.m and 4.15 p.m to get the desired effect - at the moment.

stevie - this phenomenon occurs even though I doubt the sun passes directly over that road as its arc through the sky is usually at an angle.

brachiopod - i agree that the answer to my question about it being sufficient information is "no". I do think that despite the road in question neither being on the equator nor running in an exact compass direction it is however possible to have my shadow directly in front of me. If I leave 5 mins earlier the shadow is clearly slightly to the left, whereas if i leave 5 mins later than usual the shadow is slightly to te right so there must be a point when the shadow is EXACTLY in front of me.

I'm still pondering.

And taking a greater than usual interest in my shadow while we have all this sunny weather!

[mibn2cweus]

This chart
shows the azimuth (angle to horizon from north) of the Sun for various times at a specified latitude. An azimuth of 90/270� in the morning/evening would mean your shadow would be cast due west/east respectively. Zero degrees would be north and 180� south and of course your shadow would be in the opposite direction