International Observe the Moon Night is an annual worldwide public event that encourages observation, appreciation, and understanding of our Moon and its connection to NASA planetary science and exploration. Everyone on Earth is invited to join the celebration by hosting a public or private event or attending an International Observe the Moon event – and uniting on one day each year to look at and learn about the Moon together. October 5, 2019 marks the 10th annual worldwide celebration of the Moon. Since 2010, an estimated 1.4 million people from across the globe have participated in International Observe the Moon Night events at museums/science centers, libraries, planetaria, observatories, K-12 schools, universities, public parks, community centers, private residences and businesses. Event hosts along with their partners help support and achieve global participation in International Observe the Moon Night. Between 2010 and 2018, a total of 5,551 events were registered worldwide. Over the last nine years, International Observe the Moon Night events have taken place in 107 different countries. Results of the evaluation of International Observe the Moon Night shows that the events continue to be successful in raising visitors’ awareness of lunar science and exploration, providing audiences with information about lunar science and exploration – along with access to lunar mission data and science results, inspiring visitors to want to learn more about the Moon, and providing connections to opportunities to do so. Each year, formative evaluation is used to make recommendations to the International Observe the Moon Night coordinating committee to make iterative changes for the next year. Worldwide evaluation data collection has included registration data, host surveys, visitor surveys, and social media data. The evaluation has been revised over the past ten years based on evolving program goals and resources available. Data collection has also included visitor interviews, observations, and alternative data collection strategies. We will discuss the challenges and successes of different evaluation strategies in evaluating the success of this program. Additionally, we will discuss the challenges and lessons learned in evaluating a large science outreach program that spans across so many countries and sites.