Is tonight the start of a new Islamic month?

more or less.

depends on the moon. but i guess so because it is 29th in some places

give or take a day, depending on how particular locales determine what counts as an acceptable sighting (or if they accept calculations instead of observation).

Rabi-ul-Awwal

also, technically not until sunset.

(if you're following the traditional Islamic definition of "day")

That's what I meant

I was just wondering since a Jewish month starts tonight, and it's also a lunar calendar

Do the Jewish and Islamic months always start on the same night?

the Islamic month traditionally starts on the sighting of the first crescent after a new moon.

if the crescent can't be sighted for whatever reason (i.e. cloudy evenings) the previous month is normally continued until 30 days based on hadith.

there is some difference of opinion about how that applies globally. i.e. if a crescent is sighted in one country, does it apply to everywhere (even if the crescent is technically impossible to view in other parts of the world until the next day) or not.

not sure exactly how the Jewish calendar is calculated, but since it's still based on the new moon they'll probably coincide more often than not.

although the months themselves won't always line up the same, since the Jewish calendar uses an intercalary month and the Islamic calendar does not.

so, while Jewish months always roughly land around the same time during the solar year, the Islamic months will always end up a few weeks earlier.

Right, the Jewish calendar is corrected for the seasons

The new month was historically determined by the sighting of the new moon, but since the exile it is calculated

So I guess it won't always be on the same day

Does that mean that Muslims in Chicago and Muslims in New York might have different months?

How does that work for observance of holidays?

@Daniel yeah; it basically depends on what system the community decides to use.

we tend toward the "Whatever day it is in Mecca" method at my masjid, particularly so the Eid al-Adha celebrated locally coincides with the actual day of sacrifice of the Hajj.

there is at least one hadith where a man was travelling, and had seen the crescent while away, but when he returned to Medinah and reported the sighting, they kept to the local sighting which was a day later.

so the idea that two cities could start their months on different days definitely has precedent.