Consider adding ruby-ephem excerpt options to SDK.open

[trevorturk]

This is a bit of a random idea, so please feel free to close if unhelpful. I was thinking about how it takes manual work to update the data file and deploy it, so you need to keep a bit of a wider window. So, I was wondering if we could consider adding dates and segments to the open/load_file command in Astronoby. Then, for example in my case on Heroku, I can count on the app being rebooted at least once every 24 hours. So, I could implement some logic to load the file data (into a constant to keep it in memory) but only for today/tomorrow. This might help to improve memory usage and speed, I think, since you wouldn't load the entire months/years of data you have in the actual data file into memory.

I'm not sure if it's feasible, but my thought was to mirror the ruby-ephem excerpt options, if that makes sense.

Also as an aside, I loved the Bike Shed interview! I thought you might get a kick out of seeing https://github.com/Shopify/measured if you haven't already.

[rhannequin]

I think the idea is interesting, this could be a good way to optimise the ephemeris for each body. Although for Astronoby some calculations require multiple targets, and the Earth (399) is always needed as the library is geocentric.

I don't have the capacity to work on this immediately, but I can think of a workaround for your use case. You could benefit from the app being rebooted every day to generate the ephemeris on start and store it in /tmp so that the server is never polluted with useless excerpts.

today = Date.today
tomorrow = today + 1

command = "ruby-ephem excerpt --targets 3,10,301,399 #{today} #{tomorrow} /path/of/original/de440s.bsp /tmp/de440s_today_tomorrow.bsp"

system command

This generates a 70 KB file, so yes indeed it can stay in memory for as long as the server is running.

Let's see what we got from it:

puts spk = Ephem::SPK.open("/tmp/de440s_today_tomorrow.bsp")
# SPK file with 4 segments:
# 2025-05-21..2025-06-06 Type 2 Solar System Barycenter (0) -> Earth-moon Barycenter (3)
# 2025-05-21..2025-06-06 Type 2 Solar System Barycenter (0) -> Sun (10)
# 2025-05-21..2025-05-25 Type 2 Earth-moon Barycenter (3) -> Moon (301)
# 2025-05-21..2025-05-25 Type 2 Earth-moon Barycenter (3) -> Earth (399)

The date ranges are a bit larger than what we requested, I will need to investigate that at some point, but it's not too much compared to a whole year.

And I believe you're right, that's currently the best we can do in terms of speed of access and memory usage, when dealing with excerpts.

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Thank you for the kind words. I didn't know measured and I have to say I'm a little bit jealous because this is the kind of library I would have loved to build, so maybe I can see how I could contribute.