When it comes to observing the planets from Earth, there are two that can be seen 'in transit' - passing in front of the sun. These two planets are Mercury and Venus.

Mercury, the closest planet to the sun, is the most frequent planet to be seen in transit. This occurs around 13 times per century, with the last transit happening in 2019. The next transit of Mercury is set to occur in 2032. When Mercury passes in front of the sun, it appears as a tiny black dot moving across the sun's surface.

Venus, the second planet from the sun, is the other planet that can be seen in transit. Transits of Venus are much rarer than those of Mercury, occurring in pairs eight years apart with over a century between each pair. The last pair of Venus transits took place in 2004 and 2012, with the next pair not expected until 2117 and 2125. During a transit of Venus, the planet appears as a larger black spot moving across the sun's disk.

Both Mercury and Venus transits are significant events for astronomers, as they provide valuable opportunities to study the planets' atmospheres and surfaces. By observing the way these planets interact with sunlight during a transit, scientists can learn more about their compositions and properties.

For amateur astronomers, witnessing a transit of Mercury or Venus can be a rare and exciting experience. It requires special equipment, such as solar filters, to safely observe the event without damaging your eyes. But for those who are able to catch a glimpse of these transits, it can be a truly awe-inspiring sight to see another planet passing in front of the sun.

In conclusion, both Mercury and Venus are planets that can be seen 'in transit' from Earth, passing in front of the sun. These events are rare and fascinating opportunities for astronomers to study these planets up close and for amateur stargazers to witness a unique celestial phenomenon.

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