Mercury and Venus are the two innermost planets, ordering the sun at just a third or two thirds the distance we do respectively. I’ve decided to combine them into one article as they both perform the same astronomical acrobatics, but they are by no means similar planets.

Mercury is a small rocky world, only over a thousand kilometres wider than our moon. It’s also similar in its colour, heavily cratered surface, and almost complete lack of atmosphere. It turns on its axis once every fifty eight days, and completes an orbit of the sun every eighty-seven (the fastest orbital speed of all the solar system’s planets.)
Venus is another rocky world, only very slightly smaller than Earth, by little over six hundred kilometres in diameter. It has a thicker atmosphere than our own, at some points with clouds of sulphuric acid – so not quite as friendly as Earth. Due to these thick clouds, the surface is unobservable, from anywhere other than below them, so the only photos taken of the surface have been from lander probes. The Russian Venera probes all had problems with cameras and lens caps – either with the lens caps not deploying on some or all of the landers, or in one case where it fell directly under the reach of a surface-testing arm, meaning the lens cap was tested, rather than Venus’ surface.
Venus spins on its axis once every two hundred and forty-three days, but completes an orbit every two hundred and twenty four, meaning its day is longer than its year. These two worlds rotate so slowly that if you were able to walk on the surface, you could outpace the sun and stay ahead of sunrise at regular walking speed.

Mercury and Venus can both be observed with the naked eye. Venus is well known as the brightest object in the sky, and as such, can be seen even when accompanied by the sun. However, it’s often very difficult to make out in daylight. It can be spotted during sunset, and will make itself very visible once night falls.

Venus appearing just a little while after sunset.

Mercury is slightly more difficult to observe. It’s very noticeably dimmer, and can only be seen during, or just after sunset, due to its close proximity to the sun. It’s not always further away than Venus, but it is smaller, making observation via naked eye viewing or telescope a little bit more difficult.
If you’re attempting to view either of these bodies in the daytime (or rather, when the sun is visible – sadly, daytime doesn’t make us immune to cloud) it’s important to be careful not to look at, or point your scope at the sun. For telescopes, make sure they’re properly secured, and make sure the sun won’t move into view. You can do this with apps such as Stellarium, or use specific lens caps or shades to block the sun completely.

Similarly to the Moon, Venus and Mercury have visible phases, albeit ones that don’t make any noticeable difference in detail. They work in a similar way to lunar phases, except considerably slower, and can be observed with amateur telescopes, Venus’ more easily so.

Mercury in a gibbous phase.

Mercury and Venus also create an astronomical phenomena similar to an eclipse, in that they pass in front of the Sun. These events are called transits. Transits occur when one celestial body passes in front of another. In this case, Venus and Mercury will (rarely) pass in front of the Sun. These transits last several hours, but are difficult to observe as they require both a telescope, and a solar filter to decrease the brightness of the sun significantly. During a transit, the planet can be viewed as a small ‘dot’ slowly passing across the solar disc. Sadly, transits aren’t common. Transits of Mercury occur once every two years or so, the next being in 2019. Transits of Venus are significantly more rare – the last was 2012, and the next won’t be until 2117. If you’re alive to see it, I’ll be very impressed.

Mercury and Venus are both interesting worlds in terms of features and observation. They offer a lot of potential for future space travel, and both would require the use of interesting concepts and technology for colonisation. Kim Stanley Robinson’s 2312 offers up some amazing ideas about journeying to these two worlds (and the rest of the solar system), so I really recommend reading it as a supplement to viewing them yourself.
In the meantime, we’ll have to settle for watching them from the ground. Venus is very easy to see, and I don’t doubt you’ll have noticed it in the sky countless times. Mercury proves a little more difficult, but it’s no surprise that the Sun’s closest companion doesn’t make for the easiest viewing. From the ground, we can see the both of them as bright stars following the Sun – but they’re some of the easiest objects to observe and be able to see entirely different worlds. Granted, they’re worlds that could melt you in two equally horrible ways, but also places that one day, with a hell of a lot of engineering, could be our new homes.

These objects are two that I’m less familiar with observing, so in order to gather some of the information and images, I talked with a few people on the Astrophotography and Co. discord server. They were a great help, very friendly, and if you’re interested in astronomy I really recommend checking it out.