Over February – March 2021, the planet Mars will be passing south of the beautiful, blue open cluster M45, also known as the Pleiades or Seven Sisters, making for a fantastic night-sky observing or astrophotography target.
The best time to see it will be 1-5 March, with closest approach on 3 March and the Moon nearby on 19 March, but you can start observing the two as we approach the end of February.
It would also make a good composite astrophoto showing the planet in motion across the night sky.
In this guide we’ll reveal when this Mars-Pleiades conjunction is happening, what you can see in the night sky leading up to it, and dates for when it will look at its best.
Mars’s mag. +0.5 brightness at the start of February was a far cry from the brilliant mag. –2.6 it got to in October 2020, yet the Red Planet will still be interesting to watch with the naked eye over February to March because of this close encounter with the Pleiades cluster.
Observing Mars in February 2021
At the start of February 2021 Mars was in southern Aries, approximately mid-way between Hamal (Alpha (α) Arietis) and Menkar (Alpha (α) Ceti). As February slips by, Mars nudges further east.
On the evening of 16 February, mag. +0.7 Mars passed 27 arcminutes south of mag. +4.3 Delta (δ) Arietis. A 40%-lit waxing crescent Moon joined Mars on the evening of 18 February, the Moon’s disc being 5˚ below the planet early in the evening.
Mars moves from Aries into Taurus on 23/24 February but is unable to reach its highest position in the sky, due south in darkness. Despite this, it remains the most northerly planet, managing to appear 55˚ up as darkness falls.
As we approach the end of February, Mars appears near the Pleiades open cluster. Early evening on 28 February the mag. +0.9 Red Planet sits just 3.4˚ south of this beautiful cluster.
Observing Mars in March 2021
At the start of March, Mars is located 3˚ south of the Pleiades. As it tracks east, Mars also moves slightly north against the background stars. This apparent motion means the planet appears closest to the cluster on the evening of 3 March, the separation distance being around 2.5˚ on this date.
Mars will appear at mag. +1.0 on 3 March, and its proximity to the Pleiades will make for a wonderful target to observe in the night sky during the early part of the month.
How to observe the Mars Pleiades conjunction
Binoculars are perfect for watching this conjunction. Mars and the Pleiades will easily fit in the same field of view for typical amateur mid-power binoculars.
In addition, the large size of the Pleiades suits such an instrument. Where a telescope tends to look through the Pleiades, binoculars reveal many of the fainter stars associated with the star cluster while managing to contain it all in a single view.
The conjunction will also be attractive for astro imaging: both objects are bright enough to register on many devices ranging from some smartphones to DSLRs and MILCs. A lens of 200mm focal length combined with a non full-frame DSLR (eg APS-C) will record both objects well.
For more on this, read our guide on how to photograph a conjunction.
Enter the Hyades
As we head into the second week of March, Mars will have moved east far enough to sit between the Pleiades and the V-shaped Hyades.
The Hyades is an old cluster, the closest to Earth at a distance of 150 lightyears. Estimates put the age of the Hyades at 625 million years, and this explains why its stars appear less vibrant than the 100 million-year-old Pleiades.
The distance to the Pleiades is around 444 lightyears, making it appear far more compact than the spread-out Hyades.
A 50mm lens on a non-full frame DSLR will capture both clusters and Mars in the same frame. For an added bonus, on 19 March a 32%-lit waxing crescent Moon will sit 2.3˚ to the south of Mars, between the planet and the northmost star in the main Hyades pattern, Ain (Epsilon (ε) Tauri).
Did you manage to observe or photograph the Mars-Pleiades conjunction? Let us know by contacting us via email@example.com.
Pete Lawrence is an experienced astronomer and co-host of The Sky at Night. This guide originally appeared in the February and March 2021 issues of BBC Sky at Night Magazine.