"It was a gamble, but it worked," said United States Senator Gaylord Nelson about the first Earth Day, which he founded back in 1970. No kidding. Today, Earth Day, which falls on 22 april, is celebrated in almost 200 countries and is the single biggest day in the sustainability calendar. With more than a billion people now participating in Earth Day activities each year, it is the largest civic observance in the world and is widely credited with launching the modern environmental movement.  Whether greening schools or promoting green economic policies at home and abroad, all of Earth Day's activities aim to inform and energise populations, so they will act to secure a healthy future for themselves and their children. Indeed, the remit is broad, with the definition of "environment" including all  issues that affect our health, our communities and our surroundings, such as creating green jobs and investment and promoting activism to stop air and water pollution.  This year, the focus is on creating one billion acts of green. "The idea is to inspire and reward simple individual acts right through to huge organisational initiatives that further the goal of measurably reducing carbon emissions and supporting sustainability. We want to register one billion actions in advance of the Earth Summit in Rio in 2012," explains John Maleri, who is in charge of Earth Day outreach across Europe.  He is optimistic. "at over 95 million actions to date, a Billion acts of Green - the largest environmental service campaign in the world - is steadily building commitments by individuals, corporations and governments in honour of Earth Day," he says.  The official website, www.billionacts ofgreen.org, quantifies acts of green though an easy-to-use online registration tool. It shows a vast range of pledges, including helping to green an area outside a college, not using electricity for an hour a day, eating more local food, changing light bulbs to energy saving ones, refusing to use toxic cleaning products, bringing reusable shopping bags to the shops and organising a community event encouraging others to go green. any such action counts in the march towards the target. For people struggling to think of something to do or who want to be part of a collective activity, the website features an event locator where people can find out information on any Earth Day-related activities in their area.  Indeed, the Earth Day Network has formed partnerships with organisations up and down the country, such as Trees4Scotland, which is promoting its 22 Days of Green event by doubling their tree planting efforts throughout the month, and the UK Youth Climate Coalition, which is campaigning to reduce CO2 emissions.  Companies are also working hard. Throughout april, for example, Disney Store is enabling customers to buy the new Earth Day shopping bag, raising money for the Woodland Trust. The new design features Pooh, Piglet and Tigger hugging a tree. They are on sale for ?2.50, with ?1 from each purchase being donated to the Trust. "Over the past two years, Disney has raised over ?213,000, which is helping to create a Magical Wood at Heartwood Forest," says a spokesperson.  Other companies, such as Citrix Online, have several activities lined up. "We are inviting external exhibitors to educate employees about ways to become more green," says a spokesperson. "We also have internal exhibitors with information about how to be more green in your work life. Finance, for instance, has a table inviting employees to sign up for paperless pay slips, while community connection has a table teaching employees about local environmental non-profit-making organisations where they can use the volunteer day they're given each year to help out. Facilities has a table reminding people of what they can recycle - it's not just cans and plastic bottles."  among the charities getting involved in Earth Day this year is Compassion in World Farming. "Our big hope for Earth Day is that more people than ever before see factory farming for what it is," comments CEO Philip Lymbery. "Our aim is to get people to recognise the huge damage it does to animals, people and the environment. Important Earth Day pledges could include going meat-free for a day, committing to be meat-free one day a week or switching to food raised humanely and sustainably, such as free-range."  Earth Day was born out of the student anti-war movement. Inspired by what it achieved, then Wisconsin governor Nelson realised that if he could infuse that energy with an emerging public consciousness about air and water pollution, it would force environmental protection onto the national political agenda. "Senator Nelson announced the idea for a 'national teach-in on the environment' to the national media," says Maleri. "He persuaded Pete McCloskey, a conservation-minded Republican congressman, to serve as his co-chair and he recruited Harvard graduate Denis Hayes as national co-ordinator. On 22 april 1970, 20 million americans took to the streets, parks, and auditoriums to demonstrate for a healthy, sustainable environment in massive coast-to-coast rallies. The first Earth Day led to the creation of the US Environmental Protection agency and the passage of the Clean air, Clean Water, and Endangered Species acts."  as the years went on, Earth Day continued to achieve great things, and as 1990 approached, a group of environmental leaders asked Hayes to organise another big campaign. "This time, Earth Day went global, mobilising 200 million people in 141 countries and lifting environmental issues onto the world stage," says Maleri. "Earth Day 1990 gave a huge boost to recycling efforts worldwide and helped pave the way for the 1992 United Nations Earth Summit in Rio de Janeiro. as the millennium approached, Hayes agreed to spearhead another campaign, this time focused on global warming and a push for clean energy. With 5,000 environmental groups in a record 184 countries reaching out to hundreds of millions of people, Earth Day 2000 combined the big-picture feistiness of the first Earth Day with the international grassroots activism of Earth Day 1990."  More than 40 years on from its beginnings, Earth Day is not without its cynics, who claim that the celebration itself has outlived its usefulness. But just last year, Earth Day accomplishments included the logging of over 40 million environmental actions towards the goal of a Billion acts of Green, from large-scale climate petition drives to city-wide light bulb change-outs to massive coral reef and beach clean-ups. Earth Day 2010 also saw the Earth Day Network partner with the Carbon War Room to convene 200 of the world's most important entrepreneurs for the Creating Climate Wealth Summit, which examined groundbreaking ways to solve climate change and create a new green economy based on renewable energy. a commitment to planting a million trees in 16 countries was also made.  In Kolkata, India, plans for a small series of sponsored events evolved into a movement across 17 cities nationwide. In China, 10 universities participated in month-long efforts to reduce the carbon footprint of their communities, with students making lifestyle changes, such as recycling and using public transportation. and in afghanistan, Earth Day Network worked with more than 40 government and village leaders across the country on environmental practices, including recycling programmes, clean water and alternative energy. There are many more examples of such initiatives.  The beauty of Earth Day, believes Richard Hebditch, campaigns director at Campaign for Better Transport, is that literally everyone can do something that counts. "Urgent action is needed to reduce rising CO2 emissions from transport in the UK, for example, and by making a simple pledge on Earth Day to reduce our transport carbon footprint and walking or cycling to work or catching the train instead of a plane, can make a huge difference."  'PLaNTING a TREE ONLY COSTS a TENNER '  For 22 days from the beginning of april, every tree that is planted via www.trees4scotland.com (at a cost of ?10 per tree ) is being matched by the organisation, which will plant another.  In the campaign from 1 april to 22 april, Trees4Scotland is getting individuals and companies alike to plant at least one tree.  "The Earth Day Network contacted us," says angus Crabbie (right), founder of Trees4Scotland. "We do a lot of work to raise awareness of the lack of woodlands in the UK generally and so they saw value in partnering up with us. For us, it's an interesting experiment to see just how much interest we can get from the general public, on an issue that has a few myths surrounding it."  People think planting a tree is expensive, but it just costs ?10, says Crabbie. "also contrary to popular opinion, it's really easy. It takes 100 years to grow a tree, it takes five minut