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Hank explores the science behind the first kiss -- and all the kisses that come after it -- and also sets you straight about the vernal equinox, what it is, and why this year's is special!

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Okay we've probably all seen it but let's just lay it out there, that black and white first kiss video with all the hipsters making out, kinda hard not to watch. It amassed 48 million views in about 2 weeks. Just by pairing up attractive supposed "strangers" to kiss for the very first time. Now regardless of whether the kissers were actually actors or what the video was trying to sell, it seems to have reminded the whole internet that kissing is kinda hot. And we've talked about kissing before here on SciShow, aside from viral marketing it seems to be good for a number of things among them letting us get a big close whiff of a potential mate.

Seriously though, biologists think that kissing allows us to pick up intimate chemical cues from other people that tells us whether they're a good match for baby making. The theory is that we can actually smell proteins we all have that are associated with our specific immune systems. It's called the major histocompatibility complex or MHC and it's the closest thing there is to real human pheromones. We tend to be attracted to people with different MHCs than our own. The thinking is that people with immure systems that compliment ours, will lead to healthier offspring.

Now whether that's what we are actually seeing in the first video, I can't say, but of course kissing also just feels good. Humans are the only animals with fully exposed lips and they're packed with about 100 times more nerve endings than our finger tips. When stimulated those nerves can trigger a release of dopamine, oxytocin and serotonin, chemicals that allow us to feel physical pleasure. So it just makes sense that kissing would be, you know, exciting.

But the study of kissing, it never stops, you'd be surprised. In a recent study at Oxford University psychologists went deep into how people really use that first kiss and the kisses that come after it. They interviewed 900 adults of all ages and in different kinds of relationships about how often they kiss and when and why and what it did for them. Results confirmed that psychologically as well as biologically, kissing seems to be important for mate selection. The more selective that people said they were and the more attractive they thought they were the more they said that kissing was important to them. But smooching was even more important as a way of maintaining long term relationships, for example people linked for frequent kissing with having a better relationship but this was not the case for having more sex. The funny thing is few people seem to actually think that kissing was arousing and they tended to not associated with the act of sex itself.

Now for many creatures from hamsters to horses, springtime is the official season of love. Here in the northern hemisphere it began with the vernal equinox March 20th around 11 am Eastern Time. You'll note that the equinox is not a day, it's a specific point in time when the center of the Sun crosses the plane of the earth's equator, sometimes called the celestial equator. And even though the name equinox, Latin for equal night, suggests that day and night are the same length everywhere that is not true either. That happens that different times in different parts of the world, depending on your latitude.

What's particularly interesting about this year's vernal equinox is that from now on and for the rest of the century it will fall on either March 19th or March 20th. Back in the 20th century it often fell on the 21st, which is why a lot of people always assume that March 21st is always the first day of spring. But it will not fall on the 21st again until well into the 22nd century. Why? Well the main reason is that the Earth orbit actually takes 365.242 takes to complete, while the calendar year has only 365. So as years pass our calendars keep getting behind by a fraction of a day.

That's why we have leap years to help reset our calendars to match with physical reality. The year 2000 was a bonus leap year, that happens only once every 400 years because it was a century year that was divisible by 400 and that helped move the equinox up, which is what we are seeing now. But the year 2100 will not be a leap year so without that extra day the equinox will start pushing back to March 21st around 2102. So, you know, plan accordingly.

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