When is Milky Way season and what time can you see the core

What Milky Way season actually means

When photographers talk about Milky Way season they mean the months when the galactic core, the bright, dense centre of our galaxy, is above the horizon while the sky is fully dark. The core lies roughly 26,000 light years away in the direction of the constellations Sagittarius, Ophiuchus and Scorpius, at a declination of about minus 29 degrees, according to the standard coordinates listed for the Galactic Center.

That southern position explains most of the season. Because the core sits well south of the celestial equator, it climbs high for observers in the Southern Hemisphere and stays low over the southern horizon for observers in the north. It also means the core has to share the sky with the sun: for part of the year it is only above the horizon during daylight, and no amount of planning will show it then.

Month by month in the Northern Hemisphere

Sources agree on the shape of the season but draw the edges differently. Milky Way Forecast puts the core season at roughly February to October with the peak from late May to mid-July. AstroBackyard names May to August as the best months, with June and July the strongest, and treats November to January as the off season, when the core stays below the horizon at night.

The hour changes through the season. Milky Way Forecast gives these approximate windows: about 4:30 to 6:00 a.m. in February and March, 2:00 to 5:00 a.m. in April, midnight to 4:00 a.m. in May, 10:00 p.m. to 3:00 a.m. in June and July, 9:30 p.m. to midnight in August, 8:30 to 10:30 p.m. in September, and a short window around 8:00 p.m. in October, right after dark.

AstroBackyard describes the early-season core as rising just before sunrise in the southeast, with shooting usually between 3:00 and 5:00 a.m. By late spring it is up for hours after sunset, and in September and October it sets earlier each night, so the shooting moves into the early evening. Treat all of these times as a starting point. Your longitude within a time zone, daylight saving time and your latitude all shift them.

The Southern Hemisphere season

South of the equator the season is longer and the core climbs higher. Milky Way Forecast gives a season from about mid-January to mid-November, with the best weeks from May to July, while AstroBackyard says the core can be seen from January through November and is best from June through August.

Taken together, the southern winter months of roughly May to August are the prime window. AstroBackyard notes that in many southern locations the core can pass nearly overhead, which gives far more framing options than the low arc northern photographers work with.

How dark the sky has to be, and what the moon does

Sunset is not the start of the night. The US Naval Observatory defines astronomical twilight as ending when the centre of the sun is 18 degrees below the horizon, the point after which scattered sunlight is fainter than starlight. Milky Way Forecast puts that at roughly 90 to 120 minutes after sunset, and it is the earliest moment the core shows its full contrast.

The moon matters as much as the sun. Milky Way Forecast grades a new moon as ideal, a thin crescent under 25 percent lit as excellent, a quarter moon as marginal because it washes out the fainter parts of the band, and a full moon as poor. AstroBackyard's advice is to shoot during a new moon or while the moon is below the horizon.

Because the lunar cycle is about 29.5 days, as High Point Scientific notes, every month has a run of dark nights around the new moon. A moon that sets before the core rises, or rises after you have finished, is as good as no moon for that session.

Latitude: how high the core gets, and where nights never get dark

In the Northern Hemisphere the core's highest point is roughly 90 degrees minus your latitude minus 29 degrees, a rule of thumb from Milky Way Forecast. That gives about 24 degrees above the horizon at 37 degrees north and about 21 degrees at 40 degrees north. By the same arithmetic it peaks near 11 degrees at 50 degrees north and barely clears the horizon around 61 degrees north.

High latitudes also lose true darkness in summer, which is exactly when the core is best placed. Between roughly 48.5 and 54.5 degrees of latitude, astronomical twilight lasts all night around the June solstice, which covers places such as Paris, Vancouver and parts of Germany and the United Kingdom, and farther north the sky stays brighter still. Photographers there often do better in spring or late summer, when real darkness returns.

Common Milky Way planning mistakes

Heading out at sunset. The core may be up, but until astronomical twilight ends, the sky is still too bright for it to stand out. Plan to be set up by the end of twilight, not by sunset.

Checking the moon phase but not the moon's position. A half moon that sets at 11 p.m. can leave a perfectly dark window after it. The question is where the moon is during the hours the core is up, not what phase it is in.

Forgetting how low the core sits. At mid-northern latitudes it rises in the southeast and stays low over the south, so a ridge, tree line or town in that direction can hide it. Light pollution and cloud matter too, and High Point Scientific suggests checking the Bortle scale, a rating of how dark a site's night sky is, for your location.

Working it all out by hand. Each of these checks is simple on its own, but combining the core's rise and set, the end of twilight and the moon for one place is where mistakes creep in. LightTide, for example, shows the Milky Way window as the hours when the core is above the horizon and astronomical twilight has ended, gives the core's direction and altitude at its highest point, grades the moonlight, and lays out how many usable nights each month has for the place you pick. It does not forecast clouds or light pollution.

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