New moon in southern and northen hemisphere at the same date or is there a slight difference?

Answer 1

There's no difference, with the new moon or any lunar phase.

The time of new moon is based on the alignment of the earth, moon and sun. The exact point on the earth from which you happen to observe does not change the moment that it happens.

It's much like the first day spring, which comes up (for Northern Hemisphere observers) in about two weeks. It happens at exactly the same instant everywhere in the world. However, because of the effect of time zones, spring (in the North and Autumn in the South) arrives on March 21st in the Eastern hemisphere, but the evening or afternoon of March 20th in the Western.

North or south of the equator doesn't matter to the new moon (although east or west does, at least as far as the clock is concerned). Remember, of course, that the sun always appears in the northern part of the sky from temperate latitudes of the Southern Hemisphere, like most of Australia and South Africa, and in the southern skies from the Northern Hemisphere.

Answer 2

Depends on how you define "new moon". Most all astronomers use the "Astronomical New Moon", which is defined to occur when the Sun and Moon have the same geocentric ecliptic longitude, which may not occur precisely when the Sun and Moon are closest together in the sky.

With this definition, new moon happens at a specific instant, and is therefore the same time for everybody in the world.

On the other hand, if you use an observational definition, such as for Islamic religious purposes, then you way want to know when the Sun and Moon are closest together in your sky. This could be several minutes different from the time of the Astronomical New Moon.

And, it could also be different by a few seconds for somebody in the southern hemisphere compared with somebody in the northern hemisphere, even if they are on the same geographic longitude. This is because the geographic longitudes and the celestial ecliptic longitudes are not exactly parallel.