Portsmouth Stargazing & Astronomy Report

Key Sky Quality Metrics

SQM (mag/arcsec²)

18.91

Bortle class

Class 7 (Class 7)

Darkness Quotient

32%

Dataset

July 2026

Suburban/urban transition

Portsmouth: The Practical Verdict

Portsmouth, a small city on the southern coast of the United Kingdom, suffers from high light pollution which impacts astronomical observations. The overall quality for stargazing here is poor, with suburban light sources greatly diminishing the night sky's depth. The primary limitation is the absence of the Milky Way and significant interference for deep-sky observing.

In Portsmouth, observers can focus on the Moon, planets, bright double stars, and bright open clusters. While narrowband imaging for emission nebulae is possible, broader or deep-sky targets such as galaxies and faint nebulae are best avoided. The brightest horizon is towards the west-north-west, with the south-south-east offering slightly clearer conditions.

For those in search of darker skies, an upgrade is available within 20 km east-south-east of Portsmouth, offering a Bortle 4 class sky. This site presents a massive improvement, ideal for serious deep-sky observation and photography.

At a Glance

Overall

Poor urban/suburban sky - This is a poor sky for astronomy. The Moon, planets, and a few bright objects remain viable, but deep-sky work is difficult.

Milky Way

Not visible - The Milky Way is not realistically visible from this level of light pollution.

Best targets from here

Moon, planets, bright double stars, bright open clusters, narrowband imaging with careful processing

Do not prioritise

visual deep-sky observing, broadband galaxies, reflection nebulae, Milky Way photography

Best nearby upgrade

18 km ESE sits about 18 km east south east and reaches Bortle 4, roughly 6.1x darker.

Moderate dark window

Portsmouth's longest dark windows fall in December and January, with the shortest nights around June and July. Plan deep-sky sessions around the autumn and winter months for the best combination of long nights and true astronomical darkness.

Frequently Asked Questions

Can you see the Milky Way from Portsmouth?

No. Portsmouth is a Bortle Class 7 sky with SQM 18.91, so the Milky Way is not visible from the city. For Milky Way photography, look for a Bortle 4 or darker site.

What Bortle class is Portsmouth?

Portsmouth is Bortle Class 7 (SQM 18.91), a poor urban/suburban sky for astronomy.

Is Portsmouth good for stargazing?

Not for serious deep-sky observing. Portsmouth is a poor urban/suburban sky where the Moon, planets, and a handful of bright targets are the realistic options from the city itself.

Is Portsmouth good for astrophotography?

Broadband deep-sky imaging is heavily compromised from Portsmouth and a Bortle 4 or darker site is strongly recommended. Narrowband imaging of bright emission nebulae remains viable from Portsmouth with appropriate Ha or OIII filters.

What can you observe from Portsmouth?

Primary targets from Portsmouth include Moon, planets, bright double stars, bright open clusters, narrowband imaging with careful processing. Targets such as visual deep-sky observing, broadband galaxies, reflection nebulae are not realistic from this sky.

Where are darker skies near Portsmouth?

The closest meaningfully darker mapped site is 14 km S, about 14 km south of Portsmouth, reaching Bortle 5.

When is the sky darkest in Portsmouth?

The sky over Portsmouth is darkest around January, December. Significant summer limitation: around 52 nights per year have no true astronomical darkness.

Is light pollution in Portsmouth getting better or worse?

The long-term trend for Portsmouth is gradually improving, with the sky darkening by about 0.04 SQM per year.

• 18 km ESE

  Direction

  ESE

  Distance (km)

  18.4

  SQM

  20.87

  Bortle

  4

• 14 km S

  Direction

  S

  Distance (km)

  13.5

  SQM

  20.32

  Bortle

  5

• 40 km NNE

  Direction

  NNE

  Distance (km)

  40.1

  SQM

  20.33

  Bortle

  5

• 80 km E

  Direction

  E

  Distance (km)

  79.7

  SQM

  20.57

  Bortle

  5

• 97 km NW

  Direction

  NW

  Distance (km)

  96.9

  SQM

  20.26

  Bortle

  6

• 167 km W

  Direction

  W

  Distance (km)

  166.7

  SQM

  21.05

  Bortle

  4