The midheaven or MC (Medium Coeli) is the highest overhead point at birth. As the earth rotates around its axis, this point changes throughout the day like the ascendant, and completes a full circuit of all 12 signs in 24 hours. Which means the MC (just like the ascendant) will only spend 2 hours in a sign, and is therefore extremely sensitive to birth time.
In discussing the ascendant, I presented the example of a birth at Quito, Ecuador where the ascendant changed sign in roughly a couple of hours, from 1 pm to 3 pm. Taking the same example, we see that the Midheaven at 1 pm is in Cancer, and at 3:08 pm it has changed to Leo. (If you were to draw the horoscope for 3:07pm, you would find that the MC is still in Cancer.)

I contrasted the Quito example with that of a birth in Helsinki, Finland. At the high latitude, the ascendant moved much more slowly between 1 pm and 3 pm, covering only 16 degrees of span. (So the rate of motion at that time of day was about half of that at Quito, where the ascendant had covered more than 30 degrees of span.) So here’s the Helsinki example again, this time focusing on the MC:

At 12 pm, the MC is just into Gemini, at 1o00′. At 2:05 pm, it has just moved into Cancer, at 00o11′. So the rate of motion is more the same as that at Quito, just around an entire sign in 2 hours. The conclusion is that unlike the ascendant, the rate of motion of the MC does not depend on the latitude – it is about the same at any latitude. Likewise, is it is also independent of the longitude: Quito is much further west of Helsinki, yet the rate of motion is about the same.
In sum, the MC moves at a rate of around 30 degrees (or 30*60 minutes) of arc in 2 hours (or 120 minutes) of time, which works out to 15 minutes of arc per minute of time, or 1 degree per 4 minutes. This rate of motion is independent of the place or time of birth.
The MC sign represents the social standing, status, and reputation of the individual. It has nothing to do with the personality as such, but more to do with what the person’s place in society. For instance, when a woman gets married, her status changes from single to married. The social status has changed. Likewise, when a person gets elected to office for the first time, their social standing changes. But when a person gets promoted within the same job, the status may or may not change depending on the significance of the promotion: going from manager level 1 to manager level 2 is likely insignificant and doesn’t result in a noticeable change of status, but being promoted from director to vice president might mean a big leap in the organizational ladder, and consequently a significant change in status and reputation.
The MC is also used to interpret the kind of profession, or vocation, that would be ideal fo the person. Vocational astrology is a branch that deals with profession and career, and the MC is one of the primary considerations in this undertaking. The analysis goes well beyond the MC sign itself, to include the ruler of the sign, the aspects made by the ruler, and several other factors.
Unlike the ascendant or rising sign which clearly shows in the personality, deducing the ideal profession from the MC sign is not so straightforward. However some broad generalizations can be made as a first cut attempt to narrow down possibilities. For instance, Gemini on the MC signifies a profession that would involve a heavy dose of Gemini qualities such as communication, ideas, data, etc. which could be used in activities such as writing, teaching, speaking, selling, brokering. Taurus on the MC would indicate a profession that involves Taurean significations such as art (such as drawing and painting), money (such as banking), working with soil (such as planting, gardening, landscaping),working with pretty or valuable objects (such as designing or selling jewelry).
Here are a few examples of public figures and their occupation as it connects to the MC in their horoscope.
Christine Lagarde, MC in Capricorn. Lagarde is the President of the European Central Bank, an extremely powerful and influential position in European economy and finance. Her career arc shows a steady march toward this vaunted position: Minister of Commerce, Minister of Agriculture and Fishing, Minister of Economy, Finance and Industry, Chair and Managing Director of the International Monetary Fund. The Capricorn MC is evident in this step-by-step climb of the aspirational mountain, culminating at the top positions she has held in social and political institutions. Saturn, ruler of the MC, is conjunct Mars in Scorpio, which brings in qualities of regulation and control.
Frida Kahlo, MC in Taurus. Famous artist (Taurus), renowned for her portraits, self portraits, and works informed by the nature and artifacts of her native country. Inspired by the country’s popular culture, she employed a native folk art style to explore questions of identity, post colonialism, gender, class, and race in society. The expansive exploration of a diverse set of important social issues is signified by Venus, ruler of the Taurus MC, in conjunction with Pluto (expansiveness) in Gemini (diversity).
Albert Einstein, MC in Pisces. Famous for channeling his profound imagination (PIsces) into creating theories that changed our understanding of the universe. Einstien’s Sun, also in Pisces, strongly ties in to the MC, and is in a sextile with Pluto, signifying how the imagination of Pisces was borne to lofty heights by the power of Pluto, delivering a sustained flow of supreme creativity (sextile).
Martha Stewart, MC in Leo. Entrepreneur, retail businesswoman, TV personality, former model. Published eponymous magazine, hosted eponymous TV program, created TV series named for her, which was based on her eponymous book, ran eponymous company. It’s Leo all the way (me, me, me!), with the MC ruler Sun also in Leo, conjunct Pluto (more, more, more). Her name is her brand and reputation.
Before I wrap this up, there is one special consideration that astrologers use when they interpret the MC, and that is to do with the “dual-bodied” signs of Gemini (symbol is twins) and Pisces (symbol is two fish swimming in opposite directions). With Gemini or Pisces on the MC, the individual often takes up multiple occupations either simultaneously, or in sequence. For example, one might be a university teacher and a fiction writer at the same time, or one might transition from being a lawyer to taking up fiction writing.