Seems like an Astronomer entering the game would turn it to night...
I mean, it makes sense though. The effect only triggers at change so while the astronomer arrives at day, he can only do his job when it changes to night.
As for the card itself, it's pretty deece. UR is a cool Day/Night color combo since both not playing spells during your turn (read: leaving interactive spells up) and casting two spells a turn (read: casting a buncha cheap cantrips like Izzet usually does) are very easy to achieve and filling up your graveyard is definitely something Izzet wants to do, doubly so with Flashback on the format. I'm not sure if a bear that helps you setup is where you want to be with Izzet in Constructed, but hey, Goblin Electromancer saw play and crazier things have happened.
I tried testing Suspicious Stowaway in Modern Humans. What that taught me is that switching between day and night is still surprisingly hard and will cause you to lose tempo if you opt to switch to night mode ASAP (especially if your opponent isn't cooperative). You basically cannot count on a switch to night mode until the opposing turn after the next turn after you play your day/night guy. (And, despite its similarity in looks to Ragavan, Nimble Pilferer, Suspicious Stowaway nonbos with Ragavan.)
...Which means that Obsessive Astronomer is not going to trigger digging for cards all that quickly.