• Activity
  • Votes
  • Comments
  • New
  • All activity
  • Showing only topics with the tag "astronomy". Back to normal view
    1. Astronomer here! AMA!

      I'm here on the invitation of someone else, and not sure what this new website is all about yet/ the space stuff seems pretty scant, so anyone got a question about space they need answering? For...

      I'm here on the invitation of someone else, and not sure what this new website is all about yet/ the space stuff seems pretty scant, so anyone got a question about space they need answering?

      For those who don't know me from the certain other website, I am a radio astronomer currently at the Center for Astrophysics | Harvard & Smithsonian. Known under this user account on a certain other website for comments that begin with "astronomer here!"

      122 votes
    2. Can you set a clock using a light sensor to detect sunrise and sunset?

      While pondering an off-grid microcontroller project, I got to wondering: A light sensor can obviously detect day vs night. So it could be used as a very cheap way to set a device's clock - but how...

      While pondering an off-grid microcontroller project, I got to wondering: A light sensor can obviously detect day vs night. So it could be used as a very cheap way to set a device's clock - but how accurately? To within an hour? A few minutes? How would you do it?

      Questions that arose from this include:

      • Should it detect dawn/dusk (light <-> dark transition), or noon/midnight (brighest/darkest time) ?
      • How do dawn/dusk times relate to clock time? Does it depend on lat/long?
      • If using dawn/dusk, what light level threshold to use?
      • The same threshold for dawn & dusk, or different ones?
      • Better to detect a darker threshold (start of dawn, end of dusk) or a lighter one?
      • Some days will be lighter/darker than others, so how to manage averaging of times?
      • How accurate could it be made?

      My naïve first stab at this would be: Pick a light threshold. Record the dawn/dusk times according to that threshold. Average them, call that "noon", and gradually tweak the clock time over several days to bring it into line with the sensed/calculated "noon" - but a searching for graphs of sunrise/sunset times quickly showed that the midpoint of sunrise & sunset is not noon.

      Googling threw up lots of results for sensor lights combining a clock and a photocell, but I couldn't find anything about using the photocell to set the clock. So does anyone know if this has been tried before? Is it a non-starter for some reason?

      Edit:

      Perhaps it's worth sharing the project I had in mind, which is a rain alarm so I can rush out and get the washing in from the line when it starts to rain. I was thinking how annoying it would be if I left it switched on and it rained in the middle of the night and the alarm woke me up. So I decided should automatically avoid triggering during the sleeping hours of night (say 10pm to 8am). My first thought was a photocell so it wouldn't trigger when it's dark. Then I remembered that it gets light at 3am at the moment, which wouldn't work. So it needs a clock. How to set the clock:

      • Manually - Needs a user interface with buttons and a display. Seems overkill just for a clock.
      • Serial port - Clunky to plug a laptop in just to set the clock.
      • WiFi - Needs a username and password or WPS, and an ESP32 or similar - again seems overkill just to get the time.
      • GPS - also overkill and expensive.
      19 votes