Also looks like someone at the UK's Royal Astronomical Society set up a Mastodon account and is crafting custom posts for it, but it only has ~300 followers compared with their >50K on Twitter:

@royalastrosoc

I see this all too often -- major public/nonprofit organizations set up shop here, but don't know how to best cross-link/promote it, so the accounts get kind of lost. If you care about #astronomy & like their work, give them a boost or follow :)