The Chaco Sun Dagger in New Mexico where the sun strikes the center of the engraving on the day of the equinoxes is one of the ways our ancestors kept track of the seasons, according to Krishna Mukherjee, assistant professor at Slippery Rock University. “The ancient civilizations had to figure out when to plant or harvest their crops, and the astronomical clock was the only way to do that,” she said.

Spring officially arrives Thursday.

After this winter and its unending parade of polar vortexes, it can't come fast enough.

But what really marks spring's beginning?

The return of daylight-saving time? That's a man-made event.

The weather? Hardly. Changeable weather in March is proverbial (“March comes in like a lion and goes out like a lamb.”). Last week's snow and frigid temperatures are proof of that.

What about this vernal equinox that arrives at 12:57 p.m. Thursday?

According to Krishna Mukherjee, assistant professor in the Department of Physics and Pre-Engineering at Slippery Rock University, it's all got to do with the Earth's rotation.

“The Earth's rotation axis is perpendicular to the Earth's equatorial plane,” she said. “The path that the Earth takes around the Sun is called the ecliptic.”

The plane of the ecliptic and the Earth's equatorial plane are not aligned, she said, meaning the equatorial plane is tilted by 23½ degrees to the ecliptic plane.

“If these two had coincided, meaning the Earth's spin axis was always perpendicular to its orbital path, every day the sun would follow the same path in our sky and at noon the height of the sun measured from our horizon would be the same throughout the year. This would result in no seasons,” said Mukherjee.

In the year, sometimes the Earth's northern hemisphere is tilted toward the sun and sometimes the southern hemisphere is tilted toward the sun.

“So as you move from the southern hemisphere getting more sun to the northern hemisphere receiving more sun, there is a point in the Earth's ecliptic when both hemispheres receive the same amount of sunlight. This day is called the vernal equinox.”

Traditionally, at the time of the vernal equinox spring would arrive in many parts of the northern hemisphere and hence it was given the name spring equinox, she said.“The arrival of spring, meaning when the flowers bloom and leaves appear, varies from year to year depending on how severe the winter is in North America, and it also differs from state to state in United States. Hence it is just a symbol now,” Mukherjee said.Mukherjee said equinoxes and solstices are holdovers from the past.“People had no calendars but had astronomical monuments like the Stonehenge in England or the Sun Dagger in the Chaco Canyon in New Mexico where the sun strikes the center of the petroglyph on the day of the equinoxes,” she said.“The ancient civilizations had to figure out when to plant or harvest their crops, and the astronomical clock was the only way to do that which, of course to some degree, dictates how nature behaves,” Mukherjee said.

The vernal equinox also helps determine the reason Easter is “late” this year, falling on April 20.“Easter Sunday is always celebrated on the first Sunday following the first full moon after the vernal equinox,” said Mukherjee. The vernal equinox is Thursday, and the next full moon after its arrival is April 15, pushing Easter back to April 20.And come Easter, there's an even chance the temperature and precipitation will be in the normal range, says the National Weather Service in Pittsburgh.“According to the Climate Prediction Center in College Park, Md., the three-month outlook — March, April, May — there is an equal chance of normal temperatures for that period,” said John Darnley, a hydrometeorological technician for the National Weather Service. “The normal temperature for that period is around the mid-60s.”“There's also an equal chance of there being normal precipitation for the three months,” said Darnley.That translates into 2.58 inches in March, 2.34 inches in April and 2.48 inches in May, he said.For the more immediate future, according to Darnley, in the next 10 days, “there's a 70 percent chance of below-normal temperatures and a 30 percent chance of it being above normal in precipitation.”That means gardeners need to cool their rototillers, the soil isn't nearly ready for planting yet, according to Evelyn Oesterling, the owner of Oesterling's Lawn and Garden, 101 S. Monroe St.“You have to make sure it's frost free,” Oesterling said. “It's going to take a good week of warm weather to get the ground warmed to 55 degrees.”“If you've got a clayey soil, that holds in the moisture, it's going to take longer. Sandy soil will dry pretty quickly,” said Oesterling.“Some people like to put seed potatoes in the ground around St. Patrick's Day, but the ground's not ready yet,” Oesterling said.“We've got seed potatoes coming in (this) week or the week after that,” she said, “We are taking orders for rhubarb, asparagus and horseradish roots and strawberry and sweet potato plants.”Gary Kalkbrenner, owner of Gary's Small Engine Service Center, 438 Pittsburgh Road, knows that spring is approaching by all the lawn mowers in his shop for service.“We're getting calls like crazy for pickup and delivery of mowers,” he said. “Most people are doing their servicing right now. I tell people you have to treat your mower like your car. When you take your mower in for a tuneup, we inspect the belts, sharpen and balance the blades, change the filters, change the oil, set the engine's rpms,” he added.Kalkbrenner said he and his two mechanics can have a mower serviced and back in about a week.He said people are also starting to come in to shop for new mowers, lawn tractors and weed trimmers.Kalkbrenner, who estimated his business serviced 150 mowers last year, said, “Now's the time before the grass starts to grow. That's when all the procrastinators come out.”Maybe by that time, people will have shaken off the effects of moving the clocks ahead March 9 to begin daylight-saving time.

Adding that hour of light in the evening can throw people out of sync, said Kevin Wolfe, supervisor of the sleep lab at Butler Memorial Hospital.“People with poor sleep hygiene are more adversely affected,” said Wolfe, meaning they are dealing with the inability to concentrate and general fatigue throughout the day after the time change.“It's very similar to jet lag,” Wolfe said.It all has to do with melatonin, he said, a hormone that controls sleep and waking cycles. Light suppresses melatonin, he said, so the extra light at the end of the day throws people's cycles out of whack.Wolfe said people with good sleep habits — going to bed at the same time, avoiding caffeine and alcohol at bedtime, keeping the TV off in the bedroom — are generally unaffected by the time change.To minimize the effects, Wolfe said, “Exercise helps. Getting yourself moving in the morning, exposing yourself to light, eating dinner earlier, basically tricking your body.”

A sign at at Gary's small engines in Penn twp.

Amy Hazlet of Oesterling's Lawn and Garden, 101 S. Monroe St., shows tools that will soon be needed for work in the garden.

Gary Kalkbrenner of Gary's Small Engine Service Center in Penn Township says he can tell spring is on the way because the number of lawn mower service calls goes way up.